Critical hits are achieved in this game by pressing the attack button (X) as soon as your strike lands it takes a little practice to get used to doing this but with all the enemies in the game you should unlock it naturally even if you are just smashing the attack button at random. If you are looking to grind this out the combat arena in the comic shop is a good place to do so. Also you will know it is a critical strike because Spider-Man gets these red motion blurs around his hands and feet.