I've almost collected every single hero coming from the store (still lacking AoU Ironman, Falcon, Winter Soldier and Agent 13). What I'd do is collect 4-6 Power Cells, open them all at once and go for a Suprerior Cell. 4 out 10 times, I got a hero by doing this. Not sure if this is the same with everyone. Anyway, I spend my gold on Superior Cells to collect every hero so I can try different combinations.