Once you defeat the Great Moblin at his keep, he will move to the abandoned house just one screen south of the Gale Tree in Sunken City, making bombs. Go in the house to find a pile of bombs in the corner. Set an Ember Seed on the pile or let one of your bombs explode next to it, and then the pile will blink. Get out of the house while it is blinking. The Moblin will run away and the house will explode. After blowing it up twice or so, they will catch you, knock you out inside of the house, and the explosion will kill you.