As with Classic Sonic's Challenge Acts, you'll want to finish all 45 of Modern Sonic's side missions as well. Your ranking on these doesn't matter at all, but some tips will be provided for obtaining S Ranks anyway.
Sonic Generations | Modern Sonic Challenges (S Rank)
Green Hill Zone Challenge 1: Look Out Below!
Ironically, the first Challenge Act for Modern Sonic consists entirely of 2D platforming. The catch is that a fair amount of the platforms you'll be running across will collapse after a short amount of time, so you'll have to move quickly. If you're aiming for an S Rank, the best thing you can do is practice; you'll be cutting it close with the time limit.
Green Hill Zone Challenge 2: Tails: Way Past Fast
Tails will ask for a race, with him in his airplane against you on foot. You start at around the middle of the level, and just have to make it to the end before he does. This is a lot easier than it sounds, and you've got a good chance of S Ranking it on your first try.
Green Hill Zone Challenge 3: Doppelganger Race!
This type of Challenge Act is pretty much just a time attack, but with a time limit, since you obviously need to beat the doppelganger to the end of the level. Since getting an S Rank on the actual Act 2 of Green Hill Zone is very easy, there shouldn't really be any reason for you to be extremely good with the level, so finishing under 02:20 for an S Rank here might take a few tries depending on how familiar you are with this Act already.
Green Hill Zone Challenge 4: Jump Rush
You'll now play through a souped-up portion of the level, which now has a bunch of ramps and obstacles scattered across it. Playing through it can be pretty fun, but the S Rank is actually a bit challenging. You'll need to become familiar with the new structures and learn when to use them, or avoid them.
Green Hill Zone Challenge 5: 300% Boost Gauge Challenge
The last Green Hill Zone challenge will be a bit different in that completing it will unlock a Skill for sale. You start off about halfway through the level and need to get near the end, but there are now two conditions. The first change is that the level has slightly changed, which is to be expected; the second change, which is the point of this Challenge Act, is that your Boost Gauge is three times as big as usual, but it can't be refilled. You're going to have to boost carefully so you still have a fair amount as you progress through the level.
Completing this Challenge Act will unlock the Boost Gauge Up Skill in Omochao's shop, and it costs 2200 points. As you could easily have guessed, this Skill increases the length of your Boost Gauge.
Chemical Plant Zone Challenge 1: One Ring Challenge
At the start of this challenge, you will be presented with a single ring. This is going to be the only ring in the entire level, so it'd be wise to take your time and/or be aware of your surroundings. If you have hints from Omochao turned on, he'll say something about taking good care of the ring you have; despite what he says, you can lose the ring and still finish the level, it's just going to be a lot more dangerous. As you might imagine, finishing under the S Rank time constraints is very frustrating.
Chemical Plant Zone Challenge 2: Doppelganger Race!
The typical "speed run" challenge, which is present in every Zone. If you've played this level enough times and have already gotten the Red Star Rings, you should know the level by now to the point that the doppelganger isn't even an issue.
Chemical Plant Zone Challenge 3: Amy: Taking the Leap Together!
Amy's going to help out here, and you can ask her to use her Piko Piko Hammer to send you flying into the air. Call on her whenever you need to, and then get the timing right on the meter that appears to launch yourself upward. There's a 20 second time bonus just under the goal ring, so if you're going for an S Rank, you can take a bit of a breather.
Chemical Plant Zone Challenge 4: Grappling with Grabbers
For this Challenge Act, a bunch of those spider enemies (Grabbers) are going to be all over the place, and the level is going to have a few different structures compared to how it usually looks. Getting through to the end isn't really asking for much, because it's a very simple layout.
Chemical Plant Zone Challenge 5: Underwater Ring Machine
In the Mega Mack (the pink liquid) of the chemical plant, a friendly sprinkler machine will be tossing rings all over the place. You're going to need at least 100 by the time you get to the goal ring, so make sure you pick up as many as you can and watch out for spikes and enemies. The S Rank here is definitely the hardest one so far; the sprinkler moves faster or slower depending on whether or not you're ahead of it, so you have to find a balance of staying behind and speeding up to collect your 100 rings within the time limit.
Sky Sanctuary Zone Challenge 1: Knuckles: Buried Treasure
Knuckles, an expert at finding things buried underground, has modestly hidden a bunch of medallions with his face on them, and will offer to help you find at least 35 of them to complete this challenge. Just call on him whenever you're above a patch of buried medallions, and he'll bring them up to the surface for you. The markings on the ground will let you know whether there's one, three, or five all within the same hole, and there are plenty in the level, so don't bother with any single medallions unless you're somewhere close to having all 35. If only he was this good at finding things in the Sonic Adventure games. There are also some time bonuses stuck underground as well, but again, those are only going to be useful if you want to get a good ranking on this mission.
Sky Sanctuary Zone Challenge 2: 200% Boost Gauge Challenge
This is going to be just like the challenge of the same style from Green Hill Zone. The sanctuary will have a slightly different layout to keep you on your toes, and due to the weird nature of the level, you probably won't have enough room to use most of your boost unless you really try to waste it all.
If you didn't finish the 300% Boost Gauge Challenge from Green Hill Zone, finishing this Challenge Act will probably unlock the Boost Gauge Up Skill in Omochao's shop as well. You'll have to finish both of these challenges anyway, so it doesn't matter all too much.
Sky Sanctuary Zone Challenge 3: Balloon Bonanza
As you may have guessed by the title of the mission, a bunch of balloons are just floating around, waiting to be popped. The main idea here is to use them to get around more quickly. It's much more fun than challenging or frustrating.
Sky Sanctuary Zone Challenge 4: Doppelganger Race!
The usual doppelganger race mission. For me, my doppelganger actually got through 3/5 of the level when I finished, so it's technically "more difficult" than before, if I was still doing as well in this Zone than I was in the two previous ones. Regardless, this Challenge Act nothing to worry about.
Sky Sanctuary Zone Challenge 5: High-Speed Hijinks
Have you ever wondered how this game would be if the enemies were a real threat? Here's a little dip in the water of what that'd be like. Everything in the level; the enemies, their lasers, and the platforms (but not the timer); has been moved to double speed, turning the Egg-Robos into deadly laser turrets. Stay sharp and watch out for anything that moves, because unless it's a platform or a pole, it's probably going to be an issue. The challenge itself isn't necessarily hard, but you might die your first time through, due to not having seen the game at this high of a speed before.
Speed Highway Zone Challenge 1: Overdrift
A whole bunch of rings are laid across the roads, and the goal ring is quite a distance away. To get to it in under a minute (if you want an S Rank), you'll have to run or drift into the rings so you can refill your boost meter, and try to boost for the entirety of the challenge.
Speed Highway Zone Challenge 2: Hot Pursuit
Another sprinkler is going to drop rings all over the stage, and you need to reach the goal ring with 100 of them intact. The catch this time is that the sprinkler is going to drop considerably more bombs than it did in Chemical Plant Zone, so you'll really have to keep an eye on it.
Speed Highway Zone Challenge 3: Doppelganger Race!
Another race against the doppelganger. It should be pretty obvious by now that nothing really changes between this Challenge Act and the standard Act 2 level.
Speed Highway Zone Challenge 4: Drill Baby Drill
This is going to be the easiest of the five Challenge Acts for Speed Highway. All you have to do is stomp down onto boxes below you, while also avoiding bombs and the like. If you hit one, it's super quick to recover any rings you've lost, since there shouldn't be much room for them to fly off in anyway. It'd be very surprising if you didn't get an S Rank on your first try.
Speed Highway Zone Challenge 5: Espio: Master of Camouflage
Espio is going to challenge you to a fight, which consists of him throwing shuriken at you while remaining invulnerable until his camouflage wears off. The only way to make this happen is to attack the various spotlights around the Zone and hope that they shine on him before he teleports somewhere else. Moving platforms and other obstacles make things more interesting along the way towards the goal ring. Getting an S Rank is a pain here, but the overall difficulty isn't that bad.
City Escape Zone Challenge 1: Cream: Helping Hand
Cream and her Chao will be holding the only rings in the level, and they're also the only way to restore boost. Be very careful to avoid the bombs around town, and reach the end of the level as soon as you can; each bonus that you don't use (visible around the top left of the screen) will give you a nice five-second time bonus as you cross the finish line.
City Escape Zone Challenge 2: Dash Ring 1-2-3
There are now 35 boost rings placed around the Zone, with some appearing only after you've used the one before it. Since the one before it would send you into the next anyway, that's not really going to be a problem. You will have to go through all of the boost rings, but they're directly in the path of the Act; missing one is going to be pretty hard to do.
City Escape Zone Challenge 3: Topsy-Turvy
Replacing the boost rings from the previous Challenge Act are some giant rotating wheels, as well as some athletic poles. It's pretty straight-forward, just get through the obstacles quickly. The S Rank will take some practice, though.
City Escape Zone Challenge 4: High-Speed Hijinks 2
Like the same type of challenge in Sky Sanctuary Zone, all of the enemies and platforms will double in speed.
City Escape Zone Challenge 5: Doppelganger Race!
The fifth of nine doppelganger races for Modern Sonic. Beat the transparent Sonic and you'll finish the mission.
Seaside Hill Zone Challenge 1: Rouge: The Temptress
All of the enemies in this Challenge Act are equipped with shields, protecting them from any of your attacks. The only way to make them put these shields down is to have Rouge entice them with her looks, putting them in a trance for a few seconds. Attacking an enemy will activate an energy field, and create more platforms for you to continue. Some of these, though, can be skipped entirely by just using the amount of ground you already have to make it ahead. The last part of the challenge consists of about 15 enemies in three groups; attack them until the pyramid of energy fields below the goal ring creates a path up for you to reach the top.
Seaside Hill Zone Challenge 2: Doppelganger Race!
The usual. No tricks to this one, just get to the end before your double. I actually had a hard time with this S Rank for some reason, but again, you don't need S Ranks anyway.
Seaside Hill Zone Challenge 3: Trick Island
A whole bunch of rainbow rings and trick boosters are going to be all over the place in this portion of Seaside Hill. You just need to perform 35 tricks total, which is super easy if you just hold either 
or 
the entire time you're supposed to be performing tricks. You might even get the Trickstar achievement here.
Seaside Hill Zone Challenge 4: Surf, Sea, and Sand
For this multi-terrain challenge, you'll start off with some platforming around the water, followed by some more in a more "safe" area. After you reach the end of that, it's just a simple kart ride to the finish; the catch is that lines of spike balls are going to be posing a threat to you, and since you can't exactly go back to pick up your old rings, it's very possible to lose the entire run of this challenge here. You'll be very close to the end after you make it past a pair of fire traps.
Seaside Hill Zone Challenge 5: Terror From the Deep
The last Challenge Act of this Era is going to be a fairly simple concept, but may be a little difficult to S Rank. You just need to get to the goal ring at the end of the beach, but a lot of the water has been marked off due to giant choppers moving in to infest the once-safe area. You can temporarily boost through these places to cut corners if you really wanted to, but that can be avoided. Large trails of rings should prove as the main source of refilling your boost gauge in this mission.
Crisis City Zone Challenge 1: Power Stomp Challenge
This challenge focuses around the Power Stomp Skill, which attacks enemies farther away when you stomp down near them. You just need to traverse the course that emphasizes this Skill to pass. It shouldn't cause you any trouble.
Completing this Challenge Act will unlock the Power Stomp Skill in Omochao's shop, and it costs 800 points. When you stomp with this Skill active, a big shockwave will attack foes in a wide radius.
Crisis City Zone Challenge 2: Doppelganger Race!
The doppelganger is much better at being good at the game in the Modern Era; I personally found that he was about on par with me up until 3/5 or so of the way through the level. You should have around four minutes or more to beat finish the level.
Crisis City Zone Challenge 3: Switcheroo
This is another frustrating challenge that has an emphasis on switches. They'll generally be grouped together and have different effects on the hologram platforms in the area. The S Rank basically requires that you know what every switch does, so good luck to anyone trying to get it.
Crisis City Zone Challenge 4: Blaze: Piercing the Flames
Crisis City got even hotter, and has large columns of fire spewing out of seemingly-nowhere. Call on Blaze, and she'll be able to extinguish them for you. Unlike most friend abilities, Blaze can be quickly called upon over and over again, so keep that in mind when you run through the city.
Crisis City Zone Challenge 5: Four-Way Cannon Cavalcade
Like the first challenge for Crisis City, this Challenge Act features a common level piece that functions as a cannon only after you point it in the right direction. Finishing the level requires minimal patience (waiting for things to move out of the way), while the S Rank asks of you to move unreasonably quick. Spouts of fire make particular portions of the level very frustrating.
Rooftop Run Zone Challenge 1: Barrel Ring Bonus
Robots will be guarding golden barrels which hold around 15 rings each on average, and you've got to finish the level with at least 250 of them with you. This can be quite tedious if you're impatient or if you move too slowly, and especially so if you want to finish in under a minute for the S Rank. Try not to spend too long on collecting any one barrel's set of rings.
Rooftop Run Zone Challenge 2: Doppelganger Race!
As with the previous Act, the doppelganger has definitely stepped up its game, but for some reason always gets stuck on some part of the level. For me, he got snagged at around the 2/5 mark; it might not be the same situation for you, but if not, he'll probably get stuck somewhere else anyway.
Rooftop Run Zone Challenge 3: Rooftop Rail Grind
Sonic must have a thing for boarding in cities, because this mission consists of boarding-only gameplay (the original Crisis City had a boarding section as well). The city has been adjusted to fit the needs of a board, and has several ramps that you can perform tricks on to get much further ahead. Try to do that when you can to really make short work of things.
Rooftop Run Zone Challenge 4: Endless Boost
Before you get too excited about having an unlimited supply of boost, there's one drawback that you need to be aware of. This Skill prevents you from collecting rings, so that means that you die in one hit. You'll have to get through the entire level without being hit once. It's purely a means of trial and error, so don't feel too bad about dying and having to restart.
Completing this Challenge Act will unlock the Endless Boost Skill in Omochao's shop, and it costs 3000 points. With this Skill, your Boost Meter will never deplete, but you can't get any rings. This also means that you'll die in one hit.
Rooftop Run Zone Challenge 5: Vector: Pick Up the Beat
This is by far one of the most frustrating challenges in the game. Vector will be sending a blue note towards you, which you have to home in on and hit, so it can be sent back to him. After about eight bouts of this or so, the note will turn yellow and become faster. Eight more hits, and the note will yet again become faster, and turn red to signify this. One trick that works fairly well after the yellow section starts is standing on the ground in front of Vector and constantly performing homing attacks on the note before it goes out of reach. The drawback to this is that if you mess up, you're going to have to turn around to home in on the note really fast before it touches the ground.
While many Modern Era Challenge Acts are really frustrating to S Rank, this one is just as annoying just to complete. Fortunately, you should earn an S Rank by simply finishing the level, because the game literally doesn't give you enough time to fail unless you really stretch out the first third of the challenge.
Planet Wisp Zone Challenge 1: Doppelganger Race!
Beating the doppelganger for the last time should be around the same level of difficulty as it has been for the other two Modern Era Zones, but the real frustration comes from trying to S Rank this due to the time limit only giving you five minutes. If you want the S Rank, you'll have to know the level very well.
Planet Wisp Zone Challenge 2: Cart Challenge
For this challenge, you just need to get to the goal ring by using the carts that travel throughout the factory. This should be a simple level concept to understand, especially since you had to do this in order to finish the original Act.
Planet Wisp Zone Challenge 3: Charmy: High in the Sky
As with every character, if you haven't noticed, there is a challenge in which they help you, and a challenge where you compete against them. This is Charmy's helping Act, and he's actually pretty useful. Calling on him creates a whirlwind platform underneath you that you can use to get ahead; you can even spin dash off of it and skip a lot of space, then call on Charmy again to prevent yourself from falling into a pit.
Planet Wisp Zone Challenge 4: Action Master
After having made countless simplistic stages for Challenge Acts, the team behind this game finally got to make something crazy, and that's this level. Practically every element of the previous stages are here; wall-jumping panels, boxes, ring trails, and all sorts of other stuff makes this challenge a very high-action test of skill.
Planet Wisp Zone Challenge 5: Orange Rocket, Blast Off!
The last Challenge Act focuses on the Rocket power-up, so get used to watching Sonic slowly break through three panels. The goal ring is on top of a set of panels that moves around a giant room, with two more sets doing the same thing; you'll have to time your launch to get to the ring by breaking through the panels underneath it. While it is possible to just barely skim the side of the goal ring, this is likely not going to happen, so just focus on the panels beneath it.
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