1) Ark Enemies Stun. So for Warriors, the skill
Battle Rage is useful to break stun / paralysis. You can also have the skill
Infantry Training. If you don't have those skills, the item
Amulet of Dark Whispers is very useful.
2) Get the best pre-Ark gear you can get. For warriors this is:
Head:
Ashen Helmet, or
Tiara of the Heavens.
Body:
Myrosian Breastplate, or if you didn't find it,
Ashen Breastplage. The first has good shock resistance, the second has good fire protection.
Legs:
Leggings of the Shadows do a little better than
Ashen Leggings due to the higher shock and fire resistance.
Feet:
Ashen Boots offer the best physical defense with Stability (very helpful), but
Ruined Dragon Boots offer very good fire resistance (see note).
Shield:
Shield of the Dragon Hunter, or
Infernal Aegis, offers solid fire protection.
Rings: A pair of
Rings of Isolation will help out a lot.
Belt:
Girdle of the Warrior is the obvious choice.
Cloak:
Vorator Hide Mantle, the
Spectral Cloak is a poor substitute but the poison protection will help out in one spot of the Ark.
Amulet: As stated in #1,
Amulet of Dark Whispers, or failing that,
Half-Moon Medallion, or
Flame Pendent
3) Buy some supplies: Healing potions are obvious, but don't forget to stock up on
Potions of Shock Resistance,
Potions of Giant Strength,
Potions of Shielding.
Potions of Heroism are harder to acquire but are helpful against Ark spiders. Scrolls of Recall and Scrolls of Teleport can make leaving the Ark a quicker option (Scrolls of Teleport will let you go to New Garand, where the teleport statue will let you return quickly). Potions of Fire Resistance will help you survive a fire blast if you are prone to getting caught in them. You might need some Scrolls of Detection, too. They'll help find some of the more tricky doors.
4) Get some free supplies:
Elixers of Galade are the best Pre-Ark healing potions in the game. And they're free. You just need to farm the Sunken Citadel for them.