Minecraft wiki entry for Difficulty, associated text: This applies only to mobs attacking the player. Mobs attacking other mobs always deal the 'Normal' damage listed, regardless of difficulty.
Values for the creeper and ghast assume the player is directly adjacent to the explosion.
The damage of slimes and magma cubes depends on their size. Tiny-sized slimes, while hostile, are unable to do damage directly.
Mobs deal no damage to players on peaceful.