Mortal Shell: Enhanced Edition

I first recommend that you complete this achievement in conjunction with Forever Alone:

Doing both on a second playthrough would also be advisable, so you are very familiar with the combat and the layout of the map. If you decide to do it alongside Forever Alone, you will probably need to start a new save, as NG+ begins with you in your previous shell.
A guide for unlocking obsidian dark mode has been provided here on the Nihilist achievement page:


Now for the actual gameplay tips.
As the Foundling, you die in a single hit. Your skill in dodging, hardening, and to a much lesser extent parrying will directly affect how hard this achievement will be for you. The reason I do not recommend parrying (with the exception of a few instances) is largely because I am bad at it. If you are a parrying god, I'm sure this will all come as naturally as breathing. The Foundling form comes with an enormous stamina pool. Your dodge roll is also significantly faster and longer than most of the shells. In most instances you will find yourself able to out run or out dodge every attack that comes at you. Hardening is going to be your best friend, but in every boss fight, you should keep an eye on its cooldown, so you don't accidentally press when you meant to dodge wildly.

First and foremost: Run. Through. Everything. There are only 5 bosses that are required, and because of that everything else is entirely optional. There is no reason to risk your one health on some brigand trash because you want to look flashy. In the event that you do use your parry, I recommend using the upgrade that you get from Imrod's gland, as the damage the larva does is absurd. Glimpses and Tar will now no longer have any value for you (outside of Vlas's Shop), so the trash just isn't worth it. The only exception to this rule that I made were the archers in Eternal Narthex. If you follow my guide, you will be able to kill them in two or three hits, and killing them prevents any stray arrows from piercing you on accident.

Your first goal will be to head to the shrine to unlock obsidian dark form. I did this before activating Sester Genessa in Fallgrim Tower, but it is up to you whether you unlock her first. After unlocking the Obsidian Dark Mode, head to Sester Genessa. From here I recommend you collect the Quenching Acid in Fallgrim (and unlock the Sester if you haven't). A guide for the Quenching Acids that I used is here:
https://mortalshell.wiki.fextralife.com/Quenching+Acid

You will want at least 5 Quenching Acids, and there are two in Vlas's Shop if you are so inclined as to farm tar. I could only afford those when I went for the final boss, so, to each their own. If you have a preferred weapon, I recommend pumping the Quenching Acids into it, and just stick with that bad boy. If your preferred weapon is not the starting sword (which I stuck with purely because I did want to fight Hadern again) you will have to go to that weapon's respective zone, and since you won't be able to fast travel, you might as well do the gland boss while you are there. If you want the upgrades, Fextralife also has guides for those, but I found that I did just fine without them.

After collecting the acid, you can head to any of the bosses, but I recommend this order:
Imrod
Tarsus
Crucix


For all of the zones, it should be a mad dash for the next Sester. In Fallgrim and Fallgrim Outskirts, you are running to the zone transitions. Don't forget about the shortcut tunnels between the areas, it cuts down runtime by a lot. In each of the "temples" you are heading for one of the two Sesters (one about halfway through, and one before the final boss).

For all of the bosses, I recommend that you restrain yourself and learn to strike them once or twice when you see an opening. As you are likely familiar with the fights from your first playthrough, you will probably remember their combat. Even so, your first instinct will now have to change to dodge first, harden second, and parry if you are desperate (or skilled). As you fight them (likely over many... many attempts) you will learn when you can get away with a sneaky combo, or even when to brute force them with your masterful weapon play. My longer attacks typically consisted of two strikes, then a harden for their attack, dodge if it is a long combo, then two strikes. If they are staggered, beat them to death with your favorite beating stick. Just because you have only one health does not mean you can't be aggressive. For all of these fights, there will be one or two attacks the bosses make that you can spot easily with practice, and capitalize on all of their mistakes.

Imrod:
This guy is laughably easy. He telegraphs like it's going out of style, and has a rather amusing quirk in both phases. This being that most times, if you dodge away from his attacks and run away, he will cancel his current attack to rush you and swing his mace in a circle twice. You should ALWAYS punish him for this. Often times I will combo him into repeating it 3 or 4 times until he staggers, and queue the beatin' stick. When you inevitably smack him into the second phase, you will be faced with a slightly angrier boss than before. There are three combos you will need to look to capitalize on now. The previous trick, when he wanders off to roast a dead body, and when he kicks. The kick won't kill you if it connects, but he will stand still for you to stand up and casually assault him a few times. In the second scenario, follow his chunkiness, and smack him until the corpse is aflame and he turns to you. Then just hold harden. The corpse will fly past you, and it won't even break you out of your harden. Repeat ad nauseum until he falls over from the paddlin' you gave him.

Enslaved Grisha:
Another easy one. If you are familiar with the Grisha fights, you will likely have his parry timing down. This is one of the few bosses I encourage parrying alongside using the Imrod Riposte Upgrade as it just detonates his health bar. I caution you to dodge roll constantly when he roars, as it is embarrassing to die to the icicles when there is a perfectly good monster to die to in front of you. (Interesting note: If you do die to the icicles immediately after slaying Grisha, like as he is falling, he will still be counted as dead. Yeah, that was a blast) Specifically, you will want to harden through his un-parryable attacks (your Seal should flash to warn you) and then punish him for being so darn mad. When he slams into the ground with his claw, be sure to get behind him for the paddlin' as the claw will kill you when he frees it from the ground if you are at his head. His other attacks are easily telegraphed, and you could probably attack after all of them. Still restrain yourself though, as around half health he will frequently us his stupid fast face smash attack, which can catch you unaware if you get greedy.

Tarsus:
I don't know what I did in a previous lifetime, but I must have been horrifyingly cruel to orphans to deserve Tarsus. In a shell, this guy is a slow monotonous slog (though there is stunningly beautiful animation). As a Foundling he will haunt your nightmares. The walls kill you. The air kills you. The sword kills you. The icicles kill you. UGH. I placed him second so you can face him while still likely on the high from smacking the crap out of Enslaved Grisha and Imrod. You'll probably need that high, or a good night's rest for this boss. He has very few combos, but they are punishing if you mistime your harden, or dodge incorrectly. His first phase will always start with him sliding through you. Harden or dodge roll as he yells to avoid damage. You will likely be able to follow his teleports by your 30th death, as there are small ice crystals that appear in the air and point in the correct direction. A good set of headphones would probably help too. When he teleports he will strike twice. Run directly away from him (watch the walls) or dodge away, then dodge in to get a sneaky smack. If you get greedy, he can combo into his four strike attack (not that it matters if you get hit by the first one.) Should you decide to be greedy like this, strike twice, harden, dodge the second strike, wait out the last two at a distance, and smack him again. After his four strike combo (regardless of when it happens) always go aggressive. He will always teleport away after it, and you can hit him all the way until the particle disappear. You can usually stagger him too, which will have him reappear crouched down in front of you. That's a paddlin'. When he stabs his sword in the ground, rush in, smack him, and sprint towards the farthest wall. You can't harden through the frozen air. You also can't approach him after that attack, so wait until he starts walking again before you get close. Like I said before, the air will kill you, even when it isn't fair. When you do stab him into his second phase, you once again have to run away, as the air will still kill you. Hooray! This is where it actually gets tricky. You need to save your harden until one of two scenarios. He gets too close with his rush attacks, or most importantly, his surprise uppercuts. Those things will track you from across the room and through a dodge roll. Scary. Harden through them always, and then dodge and smack him back. During the rest of the fight however, he will usually be skating around you. Bait him into sliding away to force an attack animation (short lunge, double lunge, uppercut from hell), which will usually leave him open for another attack. When he starts swinging his sword around aimlessly, run toward the farthest wall again. More AOE death ice. With persistence, you can absolutely do it, I believe in you.

Crucix:
His first phase will have you facing two opponents in one body. Remember to harden as you fall into the arena, otherwise you'll die before facing the red giant. The little one will shoot you if you stay at range, but he can't break your harden, so just wait him out as stone. His melee combos go really fast, but usually the harden, dodge, strike attack will work on him. Be wary of when he jumps high in the air. This is a premo time to harden and smack the Tar out of him, as again the little guy's arrows won't break harden. When the second phase starts, you're in luck. It is now the "easy" phase. You will need to watch out for when he kicks at you (like Imrod) at the end of his 5 attack combo using the spear and hammer (unlike Imrod). Dodging works wonders. He will also end short combo with a spear stabbed in the ground. Both of these are good times to stab him in the face. Nowhere near as easy as Imrod, but you'll make it through eventually.

After each of these bosses you will enter the mist. During this time, all the normal really easy foes get replaced with harder, more annoying enemies. You will need to dodge roll almost constantly. Get that dodge roll combo to 100. You'll thank me when you stop getting instakilled by those gremlins when they drop out of the abyss from the trees for the 15th time. In the temples, it is once again a mad dash for the Sesters, although in Imrod's temple, I just ran through the alternate path avoiding the middle Sester.

Once you have completed the three temples you will unlock the super secret final boss. Much surprise. Many shock. His fight is long, but I would say it is easier than Crucix and Tarsus. The recommendation I have for you is just just stick to him like glue. I would usually harden through his long range slides, and every other attack, so I could just wail on him. If you are persistent, he won't spawn any adds until he starts his whirlpool attack. When he jumps away (coward) chase him down. Teach him death. Show him fear. Also, the wave attacks can be easily dealt with. You should dodge roll through the first wave, all while rushing towards where the waves originated. If he has jumped away, just harden (so the second wave passes you harmlessly) and wait out his attack while hardened. If he is still there, or he slashes at you, dodge roll through his attack, then harden through the second wave. You can also get a pretty cheeky parry on him if you notice him slowly raising his torso and head up. The timing for this parry is very forgiving, and if you just want to stunlock him for a little while in the first phase, I recommend this strategy. His face smash attack gets a lot rougher in the second half, so don't bother parrying after this point. After the whirlpool which marks his transition into the second phase (you can kill the adds he spawns to heal himself, but I never bothered) he will eventually throw in a new spin attack for flavor. He may add it before, but didn't on my playthroughs. It doesn't increase his rate of attack, so as long as you instinctively dodge roll away from or harden through his attacks, it will likely never hit you. He will also inevitably spawn more adds. These obsidian nightmares killed waaaaaay more than the boss did. Focus those chumps down and delete them from unlife. I recommend running away from the boss so you can better read his attacks, and just prepare an attack in harden and wait the adds out. They'll kill themselves on you in groups that way. When you do eventually kill big baddie, you'll be rewarded with this deliciously challenging achievement. Well done!!