Minecraft wiki entry for Advancement_guide, associated text: Kill a ghast using its own fireball to get this advancement. When a ghast shoots a fireball at you, wait until it comes towards you, and then attack the fireball to send it flying back to the ghast and kill it. The reflected fireball will roughly go in the direction the player's crosshair is pointing in, so the player can aim at the position where the ghast is about to fly to.