I've done everything over again in the game, MP, I didn't die at all in multiverse madness and NADA!
Only thing I can think of is that I was playing as Brian on my son's save as my profile when I got my 500 kill achievement. So I'm going to sit on Santa's Sweatshop and kill 500 enemies with Brian, and finish the level to see if it pops. Will report back.
It's coming back...