This is tricky because a lot of games make weak enemies hard by sending hordes of them at you. A good example of this is that I often die in Killing Floor because of the clots (the lowest health/slowest moving attackers) because they can grab you and there are lots of them.
I am currently trying to beat New York Minute Hardcore on Max Payne 3 so my biggest thing is getting killed (sent in to last stand) by a single bad guy who I failed to finish off during an attack. Since it only takes a few bullets to put Max into last stand or die, I might clear a room of all the heavily armored bad guys in one awesome move but then miss a single guy with a shotgun who just kills me because I failed to see him.
Also, I die from Goomba's far to often than I should.