I think you shouldn't play on Very Easy difficulty.
Pretty sure there's over a dozen videos of people getting headshot crits with endgame weaponry (AMR, Ballistic Fist, Knock-Knock, YCS/186) on normal difficulty and Ulysses takes it like a champ, then quickly heals it back up. On Very Hard, one shotting him might actually be impossible; not sure. Maybe a Fatman could.
The situation you're describing seems impossible by my calculations unless you use a sneak attack headshot crit with max charged Microfusion cells, or you're on very easy difficulty.