Marvel Ultimate Alliance

For the eight DLC characters in the game, you simply need to get 250 kills to unlock all three of their extra outfits and unlock their corresponding achievement. The best place to grind these out is either the Sepulcher extraction point, or any of the Asgard simulation missions. Asgard is great because you can make a team of the four DLC heroes and get all those at once since the spawns are unlimited, then just finish the mission and start it again with the four villains to do all of theirs.