Life Steal vs. Spell Vamp

Yeah, I know it's a team game, and I definitely never fight unless we are evenly matched (equal or more teammates than enemies). I try to pop out of the jungle and ambush the enemies from behind while they're concentrating on my teammates / minions, or when they're fleeing with low health. The SoloMid guide I've been loosely following is less of a tanky build.

Amumu's got a strong initiate, you're wasting it by trying to play him like an assassin imo.