Quote from oCrapaCreeper
He also applies the Wither debuff to other mobs, so he can heal by simply shooting a passive mob.
Then fight him underground, where you have the advantage. He's three blocks tall, so build a long corridor put beams over your head so that he has to stop and eat them before he can continue to pursue you. Have iron golems ready for a steady stream of damage. Regeneration potions, golden apples, eat whatever you need to survive. When it comes time to hit him have a sword enchanted with Smite to do extra damage to him. This is all in theory of course, but it's certainly possible to kill this guy. Which is good enough for me.
Quote from Xorjt
Okay so the plan is to make a pyramid of power, buff yourself with potion after potion, have a powerful diamond sword, and full suit of highly enchanted diamond armor, and get stuck in.
Bring a friend.
You can't build a pyramid of power since one of the ingredients for it (a nether star) only drops from the Wither.