Skullcrack

Effects that would replace gaining life with another effect won't apply because it's impossible for players to gain life.

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.

Skullcrack targets only the player or planeswalker. If that player or planeswalker is an illegal target when Skullcrack tries to resolve, it won't resolve and none of its effects will happen.