I managed to do this on my first attempt at Steel Soul, and was surprised that it wasn't as difficult as I thought.
As with the regular Steel Soul achievement (and eponymous game mode), knowing the game and mechanics inside & out is near-vital to be successful.
Although I didn't personally use it (I found it after finishing this achievement), Rusty's
15 tips video can help.
The extra challenge from this comes from a few things:
- Not knowing what counts as completion percentage (a full list of what counts can be found
here) and what doesn't. You can get 112% total, but quite a bit of the game's content isn't required to get 100%.
- The overall length and difficulty of the game, especially the Godhome content (I didn't do it, but it can help) {also: Deaths in Godhome don't count as actual deaths, so you can do the Pantheons if you want without fear of losing all of your progress}
- Personal stress and/or paranoia about dying & losing everything (I struggled with this for quite a bit, but if you can deal with it then it's fine)
Plus, you can do things from each "DLC pack" (which are actually all free updates included with every purchase of the game) to get the last few points needed.
Dream Warriors each count for 1% completion (7% total if all are beaten), but can be difficult due to the RNG of their attacks. I personally managed to defeat them (still not sure how lol), so if you're skilled enough and/or have good reflexes you might be able to beat a few.
Both Dream Nails count as 1% completion
each, as well as
Knowing your own limitations can be helpful as well. Being cocky is what leads to at least 85% of all deaths in every game.
Also, visit the Temple of the Black Egg as soon as you unlock it, and read the tablet near the bench to gain the World Sense ability (shows completion percentage in your inventory).
If you're at 100% or more, fight the final boss and pray you don't die.
If not, go get those last few points (I had 96% when I got there, was a tad perturbed I had to hunt down more stuff)
(Also selling relics whenever you get them is a great money-maker, which can help since you need to buy a few things for completion percentage)
A short list of what to aim for, completion-wise. These not only help boost the percentage, but can help you during the game:
- Equipment. These are all of the basic abilities you get (Mothwing Cloak, Mantis Claw, etc.), and half of them aren't even required to beat the game. You gain 2% per piece, for a total of 14% if all 7 are collected. Plus, you'll have much more maneuverability.
- Spells. The abyss versions of each count as 1% extra completion (6% total for getting the base spells and their upgrades). Plus, they help
a lot during combat. Especially Descending Dark's i-frames and insane damage output (upwards of 88 per use if you have Shaman Stone and every shockwave hits).
- Masks and Soul Vessels. The Grey Mourner's mini-quest can be a massive pain, but it's worth it. Especially if you clear the path beforehand (most enemies don't respawn until you rest on a bench, so all that's left are the small weak ones).
The extra health and soul storage is invaluable, and each full mask (and vessel) counts as 1% completion (if you get all 3 vessels and all 4 masks, you get 7% completion in addition to more survivability)
- Nail Upgrades. Not only does each upgrade boost your damage, but each one counts as 1% completion. 4% total if you get all 4 upgrades, and you'll deal 21 damage per swing (which can be increased to 31 using Fragile Strength)
- Nail Arts. The platforming can be a bit of a challenge, but each master has a bench before you meet them. Each art counts as 1% completion, and can help in combat.
- Dreamers. You're required to get these anyway, but each one counts as 1% completion. Nice.
- Charms. You can get 36% completion if you collect every charm, and most are rather easy to get. (but the ones that replace an existing one {e.g. Unbreakable Strength replacing Fragile Strength} don't give any extra)
Note that dying in the White Palace still counts as an actual death. And the Traitor Lord is a difficult fight, so getting Kingsoul probably isn't worth it (both halves need to be gained in order for it to count as a charm and thus give the 1% completion)
- Trial of the Fool counts as 1%,, but it's long & grindy so it's not worth it. If you want the
achievement, do it in a regular save.
- A bit of an add-on to the above, but doing the Trial of the Warrior and Trial of the Conqueror will help. You'll get a Charm Notch and Pale Ore, respectively. And each one counts as 1% completion.
If you do everything on this list, you'll gain 75% completion (my math might be a tad off though). That's already 3/4 of what you need, and I didn't even mention any of the boss fights that count. Nor the Godhome content.
Sorry for this massive wall of text, but in conclusion this achievement really boilers down to knowing the game.
I managed to get Speed Completion while doing this too, which was great. Three achievements at once.
PS: Ending-based achievements (such as this one) won't trigger until
after the cutscene finishes.