RE6 is full of bosses that just, won't, freaking, die. Honestly, in Leon's campaign, Simmons fought four long, grueling battles, as a dog-thing, a f***ed up T-rex, a messed up scorpion wannabe, and then a giant f***ing fly from Hell.
Then Haos, chasing, getting killed, then it chases again, gets electrocuted and killed, then it gets cutscene-killed by a shock from Zeus, er, Piers.
And then this guy! Shot full of enough bullets to melt int a lead engine block, dropped by a falling helicopter that probably blew up on him, stabbed by an industrial drill into a wall, crushed by a falling metal tower, dropped into molten metal, and then finally shot in the heart by a tandem aimed .500 Super Magnum revolver that was miraculously loaded. Capcom sure loves bringing bosses back and abusing the hell out of them. And the Ustanak, I'm glad they only had one success with that thing. I've seen some durable creatures, but the Ustanak... He redefined the term.