Basically, critical hits happen when you match more than 3 skulls. You deal the normal amount of damage +1 per extra skull. So if you were to deal 10 damage but you match 4 skulls, you would deal 11. Also, skulls do 1 point of damage on their own. So for instance, if you explode some gems and amongst them you have 2 skulls, you would deal 2 damage to the first unit. Let me know if I misunderstood the question. 