Marvel Super Hero Squad: The Infinity Gauntlet

Go through levels 6, 8, and 10 to collect the majority of the space stones. After your first play through go back into free play and play the same 3 episodes. Be sure to look for hack, wall smash, zipper rifts, and other special actions you could not use before when you just had the 2 characters. These will unlock a small area with extra stone fragments and sometimes a prize needed to complete one of your hero's feats.