Once the level starts, distract Crikey by hitting a food box with a boomerang. While Crikey is distracted by the chum, dive underwater, find an airtank (you can find it easily by looking a stream of bubbles) hit the air tank with your Aquarang until it floats up to the surface. Climb up to a platform and get to the raft closest to the air tank. Crikey will start biting the tank. Once you get a clear shot at the tank, hit it with your flamarang. It will explode, knocking off a piece of his armour and a quarter of his health. Repeat this three more times, but be careful.