Whorls are the Nautilus looking enemies in the water level (Sunken Tomb). I suggest playing Commando and picking up boxing gloves for the high knockback and low damage. Simply go to the bottom left of the level where there are three or four pillars. Stand on these pillars until a Whorl spawns. Then, you want to get behind the Whorl and shoot it until it falls down the gaps and falls off screen. Repeat 20 times.