Originally Posted by Rocket
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.
If you are going to replay a level over again. Make sure you collect all the secondary objects again and finish the level. Do not just quit out once you pop an achievement. This is why a lot of people are having problems unlocking the "There is no Cake" achievement.