DemoMan

The big bad boss of the 1st act, DemoMan is both destructive and stupid enough to warrant a state of the art armour preventing him from being blown to smithereens by his own grenades or his wind-up bombs. The Protagonist has no such safety net and has to avoid being hit. The Protagonist, on the other hand, does not have a high cholesterol level and can eat a few pizzas without rolling over and dying, making this encounter quite fair in retrospect.