Instructions
Avoid the jellyfish. This is your top priority. If it touches you, then you'll take damage. It doesn't have a ranged attack, though. Neither does it chase you. Instead, it drifts around the room at random.
Leap onto the top-most platform. You'll see three chandeliers attached to the ceiling. You kill the jellyfish by dropping chandeliers on it.
Attack a chandelier just as the jellyfish swims beneath it. If you time the attack well, then the chandelier will fall on the jellyfish and take away one of its life points. You have to hit the jellyfish five times to kill it.
Jump to the ground floor and run to the right side of the screen. You'll find a lever there. Attack the lever, and all the chandeliers will respawn. You can then return to attacking the jellyfish.