Use team mode and fight with 8 characters. Defeat the entire opponent team by only using 2 or more of your characters to unlock the 7th costume for the characters you used

Wolverine gets his original yellow/blue/black costume.
Phoenix gets a red/gold costume similar to Dark Phoenix.
Juggernaut, Gambit, and Psylocke get a costume similar to their comic book original.
Beast gets a blue costume with a yellow X on his chest, and he also has a helmet.