This achievement can be obtained during the boss fight with The Urn Witch. At some points during the fight, she will hop inside of her pot and do a couple of jump-slam attacks. Afterwards, the pot will spin around shooting purple fireballs in a circle around the stage. Line up a shot with the fireball spell and shoot it directly into the open hole at the top of the pot. It can be a little tricky to nail the shot, but you should have plenty of opportunities to do so during the fight. Landing the fireball will cause the pot to catch fire, stunning her for a bit and giving you this achievement