Slime Eel

Link encounters this boss in the Catfish's Maw dungeon in Link's Awakening. When the battle starts it will make a hole in the ground and two on each wall of the room. Now, it's tail is always spinning slowly in the center of the room, which you have to avoid at all costs. The key to defeat it is to wait until he pops out of one of the walls and use the Hookshot to pull it out and expose its heart, then slash it as many times as you can. Keep in mind that the farther you are when you pull the Slime Eel, the more exposed its left and you have more time to attack its heart. Sometimes you'll pull out a fake eel that will explode in a few seconds.