The Flood, every level that you do with flood, from the infection forms everywhere, to combat forms leaping too you from acorss rooms, and coming bacck up if they didn't take enough damage the first time, and carrier forms making more infection forms. And then in halo 3 the tank, stalker and ranged, the former 2 can be delt with enough skill, but ranged??? Pinpoint precesion, about anywhere from 3+ of them in a given area, and with what else could be in the area, best to hide and let some of them come to you, behind a wall, to thin the numbers a bit, gunshots work find for drawing them out.
Though I don't get why cazadors annoy people, often by the point I find some I have 1-2 companions, and grenade launchers work wonder of them soft bugs.
Also from borderlands, rakks though annoying can be killed with gunfire, best when in a buggy. Though i loathe loot midgets, spawn randomly, high hp and damage, and if some spawn in the mid of a battle,(hey I like looting) or when your love of shields and hp after a battle, your doomed.
Lastly world of tanks, heavy tanks, since i play german tanks, which don't get heavies till t7, two whole tiers after most of nations get a heavy tank, heck france gets their heavy at t4!, heavy will one shot you, ingore most of your shots, and laugh while doing it.