Minecraft wiki entry for Defeating_the_wither, associated text: Once the wither has dropped to half health, try to get close enough such that the player can sneak in hits with a sword. Here too the Wither should be trying to fire directly at the player, with bedrock blocks between them and the Wither. Try to hit its tail. This will trigger an animation where the Wither won't shoot, and the tail ends up being in the same location again. They can hit it over and over until it's dead.