Epic Mage Armor

You touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 20 + its Dexterity modifier. For the duration of spell, any critical hits against the target become regular hits, and it gains resistance to bludgeoning, piercing, and slashing damage. The spell ends if the target dons armor or if you dismiss the spell as an action.