First, you have to put on the aiming mode(triangle). Next,you have to shoot all the tentacles that turn light green, then a large tentacle will grow out in the middle of the stump. Go to the exact right spot so the tentacle cantry to hurt you but it can't and after the first whip, shoot a charged knockback jynx and it will blink red once and keep doing this until it looks drousey,then use the Incendio spell and the gate will open and it will die.