The following party member combinations, in any order, provide each of those characters with the bonus listed.
Blind Guardian.

Wolverine, Nightcrawler, Sunfire and Colossus activates "New X-Men". This provides an additional +15% of their maximum health.

Magneto, Juggernaut, Scarlet Witch and Toad activates "Brotherhood of Evil". This provides an additional +5% experience.

Jean Grey, Rogue, Storm and Scarlet Witch activates "Femme Fatale". This converts 5% of the damage you inflict into health.

Wolverine, Colossus, Juggernaut and Rogue activates "Bruiser Brigade". This gives a character +20 energy if their attack knocks their target down.

Change all the characters costumes to their original (for example, Magneto's red and purple outfit, Nightcrawler's outfit with the swords on his back) to activate "Age of Apocalypse", which gives +100% to your attack rating.