Minecraft wiki entry for Achievement_guide, associated text: Aim at the ghast then punch the fireball when it is close enough to you to deflect it. If you can perfectly aim, a shot arrow or fishing rod bobber may hit the fireball and deflect it, or you can hit it back with a sword. When you are hitting it you have to time it right, and to hit it back with a sword you use the use button with a sword in your hand.