A level 21 warrior is definitely capable of handling the Ark. You need a plan though.
First of all, get high Shock and Spirit protection, using resist potions so you are around 100. Hoard about 30 elixirs of Galade and get all the ashen plate pieces from the Sunken Citadel, and also the Axe of Minotaur Kings.
If you want to go 2-handed: all into STR, as high as you can, preferably up to 7-8 with items. AGI won't be that important, maybe take it to a natural 3. Make sure you max 2-handed expert and, to deal with stun and paralysis, get Combat Rage. A few points in Whirlwind/Charge help to ends fights quickly when you use a big weapon.
If you want a shield plus a light weapon: get your AGI to 6 and learn Flurry. Use an Adamantite Shortsword or some other high tier light weapon; still you need Rage to break paralysis and stun. Heavyhand and Duel are good skills for fast attackers.
If you want to keep your shield and a hand weapon (not light!): you can try to balance your STR and AGI, but AGI opens up Flurry so probably a better option. Learn Infantry Training to deal with stun/paralysis.
The ark seems undoable at first, but it isn't. There's plenty of good drops there in form of healing consummables and some great armor and weapons that will make it easier and easier. You can also rest at certain locations. Just make sure you dodge the explosions...
Thank you so much — I'll try this strategy! (In the 2-handed version)
Let me ask you this though: if I beat the Ark, which I gather is the last part of the main quest, will I be able to go back to the game and finish Horror and venture around?