Very easy to do this one. At the end of the water levels you will fight a large eel. Wait for the eel to rise out of the water and focus on the side it presents to you, making sure to destroy each 'bulb' in turn. A couple of hits from the projecticles you can fire with A will be fine.
Repeat this a second time as he flips round and presents his opposite side to you. Finally destroy the bulb found on his rear four or five times to shrink the eel down to size, and finally destroy him.
Quick tip: Holding down A allows you to auto-fire, but only slowly. You can spam A to fire as fast as you like to be sure you manage this within the sixty seconds allowed,