Oracle Text

Players can't gain life this turn. Damage can't be prevented this turn. Skullcrack deals 3 damage to target player or planeswalker.

Card Rulings

1/24/2013 Skullcrack targets only the player. If that player is an illegal target when Skullcrack tries to resolve, it won’t resolve and none of its effects will happen.
1/24/2013 Spells and abilities that would cause a player to gain life or that would prevent damage still resolve, but the life-gain and damage-prevention parts have no effect.
1/24/2013 Effects that would replace gaining life with another effect won’t apply because it’s impossible for players to gain life.
1/24/2013 If an effect says to set a player’s life total to a certain number and that number is higher than the player’s current life total, that part of the effect won’t do anything. (If the number is lower than the player’s current life total, the effect will work as normal.)