The Nameless Horror is fucking bullshit!

This boss is a damage race, in a way. You need to deal 3500 damage to him to win.
He'll deal stacking amounts of damage proportional to your current health every 5 turns, starting at turn 1. Beginning with 25%, up to 100% at turn 16, which means you die. You can stun him right before these turns to buy you five more.
As a bonus end game boss, you should be well prepared with an array of spells and abilities to take him down.

Now, for each ability:

Come Unto Your Maker:
The stacking ability. He's guaranteed to use it every 5 turns starting at turn 1, so grab a stun and use it then. Warhammer, Izma's Gauntlets, Dragonbreath, Shield Bash, all of these work. He has a small chance to recover instantly from stuns, but it's still viable.

Overwhelm:
A shitton of physical attacks at once. Very, very painful, but being physical based means there's an array of things you can do to avoid it.
Blind can save your life here.
A good shield is useful, especially if you're not using your fatigue for anything else.
The Dullahan's Counter ability is great. If you have TFs that raise speed, you can get a ~60% chance to counter his attacks.
The Obsidian Golem Full Plate armor is also good if you get hit, since he'll take damage for every strike.

Embrace Nothingness:
Denies most damage sources from hitting him for a few turns. Will screw you over if you don't have any magical attacks.
You can use this time to buff yourself(lustzerk is good, he doesn't deal lust damage) or heal yourself.
>If you have more than 90 INT, however, you can deal magic damage at 40% power.

Revelation:
Grants an intelligence bonus, but incurs a chance of being stunned every turn for three turns. This is probably his most dangerous ability at the end of the day, considering the "damage race" aspect of the encounter.
>If you use Test Resolve and get a Stalwart result, you're immune to this ability.

Refashion:
A blessing or a curse. Will randomize your stats from 40 to 250, which can either cripple you or make you obscenely powerful for a few turns.
Having physical and magical specials will allow you to make use of net positive STR/SPE/INTE results. It's also useful to have Whitefire around in case you're using Double Attack and your speed goes under 60.

Overall tips:

*Bleed deals damage proportional to max HP. Look at his max HP. You can get bleed effects from Bite(shark mouth), Hooked Gauntlets or Izma's Gauntlets.
*He's not immune to poison, so taking down his speed with a few naga bites may make it easier to counter or dodge his attacks.
*The Gore ability deals physical damage based on your overall weight(so STR, thickness and height). If you're a three meters tall bulked up monster, this ability crushes just about everything without armor.
*He has very low armor, so you can just pick whatever weapon has the biggest damage number. Good options are:
Single handed: Maxed out Beautiful Sword or Scarred Blade
	>The Cursed Dagger is obscenely strong with Tactician and high Intelligence, if you have enough health to handle it.
	>Izma's Gauntlets have lower damage, but can stun and bleed, and he's immune to neither.
Two Handed: The classic Warhammer may do well, since it can stun. The Claymore still hits like a truck.
	>The Dullahan's Scythe deals the most damage of any weapon in the game with Weapon Mastery. You can offset the damage on swing with enough regen and single attacks(which grant a STR boost on large weapons).

Ranged: The Flintlock and Blunderbuss(the armor penalty is irrelevant here) are both great options, aside from the reloading issue.
	>If you're a mage, the Eldritch Staff is pretty powerful with the basic Magic Bolt.

It might be an unknown fact, but you can check your expected per-hit damage on your stats screen. It accounts for pretty much all static modifiers you can have.

Aside from that, remember to use high tier rings, equip your best armor, get some acupuncture done, bring some heal items(god's mead or Heal Pill) and pray just a little bit. Using Test Resolve may tip the fight in your favor, and since(as of the last change to him) he's readily accessible from your camp, I think it's worth the RNG roll.