[Refplace]

Armor Piercing attacks, grappling, things that slow down his movement.
Diffuse creatures that can bypass armor, like flying swarms of bugs.
Attacks that go against Will, such as magic.
Lightning that ignores most metal DR.
Tentacled horrors that can grapple can give him a challenge even without doing massive damage.
Traps or even rickety stairs that penalize all that weight.