Minecraft wiki entry for Beating_a_challenge_map, associated text: Shooting it with a fully-charged bow (4.5 hearts of damage)
Using a potion of Harming II (6 hearts of damage)
Hitting it with a Sharpness I enchanted diamond sword (4~5 hearts of damage)
Setting it on fire with a flint and steel (natural causes)
Burning it in lava (natural causes)
Critical hits (4-7 hearts of damage)