Believe me or not, but Mass Effect 2. That final boss on Insanity with a Vanguard, stalling to pick up more heavy ammo while evading Harbingers and that massive laser that can catch you if you're on the edge of cover. Since it pretty much starts circling you you need to watch the hell out. And it's quite long, even with the Cain. Vanguard gameplay is SO bleeding edge and visceral.