This Achievement is to complete the game on Iron man mode on Classic or Impossible difficulty. This means you do not have a save system to abuse. You have your one save and it saves every turn. Which you might be thinking allows you to exploit it and you are correct. The game saves at the start of every turn (Not Alien's turns). Which means that you can ALT+TAB to the desktop and close your game at any point before your next turn starts and you will be able to start that turn fresh. Be aware the game doesnt save on your first turn, so if you use this method it will take you to Geoscape right before you started the mission.
Now I recommend Classic Difficulty because it is easier and less stupid. Make sure you select the tutorial as it helps level up your guys to start and really makes the first 2 months a lot easier. Plus you get a free Satellite. Select North America as a home base for this. This reduces the price and maintenance of aircrafts and aircraft weapons by 50%. Believe me this is much better than Africa's 30% Credit bonus.
Now the important part is the creation of your Squad. I went with 2 Snipers, 2 Heavies, 1 Assault and 1 Support. The Snipers and Heavies are essential. Having two Snipers with Squad sight and Double tap basically carries your team. Headshot can also get you a lot of one shot kills on tougher enemies. Heavies will be used for their Rocket launcher and Heat Ammo for Robotic enemies. Assault is self explanatory. I gave it all the defensive abilities to keep her alive and used her to move around and cancel out enemies Overwatch. Support Class is for healing. You are no doubt going to take damage at some point. The medic kit is super useful here. Support is also a great recon class as it gains an extra 3 tiles movement with one of its abilities which allows you to move into enemy range and pick off enemies with your Sniper's Squad Sight. Make sure you take advantage of the Ghost amour for this as well.
Here is a full list of the Upgrades used. Use Carpace Armour for all soldiers until the selected one is available
Sniper (x2)
Squad Sight - Used to pick off Enemies within allies sight radius
Damn Good Ground - +10 Aim and +10 Defense against enemies at lower elevation
Disabling shot - Enemy cant fire primary weapon for 1 turn. Useful for capturing alien.
Opportunist - Removes Aim Penalty for Overwatch and allows critical hits
Double Tap - Allows to actions to be made assuming no moves were made. 1 turn cool down.
Armour: Archangel - This unit is not usually in sight of the enemy.
Item - Scope (+10 Aim Bonus)
Make sure you upgrade the Pistol in the even you are in a jam.
Heavy (x2)
Bullet Swarm - Firing the primary weapon as the first action does not end the turn. Cant use Rocket afterwards though.
Suppression - Fires a special shot that acts as an overwatch if the target moves. Target suffers aim penalty. Helpful for Alien captures or prevent this alien from doing damage during its next turn if you wont be able to kill it
Heat Ammo - +100% damage to Robotic enemies
Grenadier - Allows 2 Grenade uses. Great for getting rid of enemy cover.
Rocketeer - +1 additional rocket. Great for long distance cover removal and deals 6 damage. 9 with the Blaster Launcher later in the game.
Item - Grenades
Armour: Titan Armour
Assault
Tactical Sense - +5 Defence Bonus per enemy in sight
Lightning Reflexes - First overwatch shot always misses
Flush - Fires a shot that forces enemies out of cover
Close Combat Specialist - Grants a reaction shot if an enemy moves withing 4 tiles of soldier. This saved my butt so many times.
Resilience - Immunity to critical hits
Armour: Ghost Armour
Item: Arc Thrower or Grenade
Support
Sprinter - Allows to move 3 additional tiles
Field medic - Medkits can be used 3 times per battle
Revive - Allows medkis to revive soldiers at 33% health rather than just stabilizing
Dense Smoke - Smoke grenades cover more area and add an extra 20 defense to units in it.
Savior - Medkits heal 4 more health per use
Armour: Ghost
Item: Medikit and Grenade (Deep Pockets)
Now for the strategy. You will get a free satellite, use it on a country in Africa. Your goal here will be to get Africa's bonus now. I was able to beat this without losing any countries. You will want to try and complete the continents as once they are done, abductions stop in those areas. Which makes it easier to manage. I was able to complete Africa then South America, then North America, then Europe and then Asia. Try to complete the missions as fast as possible until the assault mission is available. The two base assault missions are essential to keep knocking panic down as you will get some in the red at times and this mission can be done late in the month to lower panic. As per many other guides out there, always launch your satellites a day or 2 before the Council report.
It will all depend on the situation at the time but your priority for abductions should be $200 or Engineers. Always make sure you build at least 3 Satellites to start per month. They take 20 days to build so you got to be ahead of the game. In order to successfully get all the satellite coverage you will need 2 uplinks and 2 nexus' adjacent to each other in a 2x2 square. I was usually able to set one up per month. I believe you need 10 engineers for the 2nd Uplink, 30 for the first Nexus and 40 for the second. You will also need to be sure you have enough power. I believe I was fine with 3 power generators and a thermo-generator all adjacent in a 2x2 square of course. You can use workshops to add engineers but be sure you adjust your power needs.
Do not do any Autopsies except the Floater Autopsy and the Interrogations before completing South America. The bonus will complete them instantly.
Many people say skip Beam weapons but dont. They are typically quicker to research and allows you to research more important things. The weapons are strong enough to keep you heavily armed until Heavy floaters are introduced. Making sure your Interceptor are upgraded is essential. Do not worry about getting a Firestorm, you can wait until the after the Hyperwave Relay. As long as you have the dodge matrix and have upgrade your Interceptors weapons you should be fine.
The final mission is probably one of the easier missions. Move into room one slowly. There will be 2 Sectoids and a commander. You should be able to dispatch them easily. Take advantage of your snipers and take out the Cyberdiscs. Leave the drones to the rest of the unit. Spit you unit into 3 groups and head to the next areas. I went Assault\Heavy(Left), Sniper\Support(Top), Sniper\Heavy(Right). The next rooms will have 1 floater for group 1, 2 floaters for top group and 1 floater and 1 heavy floater for group 3. Once they are killed, 1 Crystallid will spawn where group 1 is and 2 will spawn where group 3 is.
The next little area towards the next room will have 2 thin men up top and 1 thin man for one of the groups below. Make use of the Sniper's pistol to dispatch both. If you manage to get lucky and the other Thin man is near you Psionic unit, mind control them and send them into the next room with the 4 muttons and a beserkers. Use him to attack one. They will kill him next turn. Next bit is tricky as the bererker can really choose any of the 3 ways. Doo your best to dispatch him as quickly as possible. If the Sniper is available, use the head shot ability.
A little further in that big room is 2 sectopods. Get your heavies around both sides and there is a area at the side where you wont activate the Sectopods but are within range with the Blaster launcher. Do this right and you will kill both with 2 Rockets. You can line everyone up top as well just in case. Make sure the Snipers go there regardless. Take your support and run forward on the ground level to reveal 2 Elite Mutton. Head shot ability should wipe both out easily. If they survive, use ghost armour to hide the support. Now the finale.
DO NOT ENTER THE FINAL ROOM. Doing so will trigger the final enemies, you do not want this.
Lineup your snipers behind the half cover right in front of the hallway. Ensure that Head shot is available for use on both snipers. Once they are set up slowly send your support into the final room. You want to activate them on your first move and then use the ghost armour. This allows you to use the Squad sight on the main enemy at the back. Use the head shot ability on him with the snipers for massive damage and you should be able to finish him off with the double tap if he isnt dead already.
CONGRATULATIONS! You completed Ironman!!