Aquatis

Aquatis is incredibly easy. Just do the following and you should have no probs killing him:

1. Clear the tentacles with magic (or if you are out of spells, with pistol)
2. Run to the edge before him. DO NOT JUMP ON THE STEP, IT WILL MAKE THINGS HARDER. Once you are on the edge, hold the dynamite button until Claw throws it himself. It should hit the hole behind Aquatis
3. Pick up the newly appeared dynamite and repeat

If you run out of both magic and pistol, just down tentacles with sword (beware of hug though)

Aquatis is one of the easiest (if not THE easiest) bosses in the game. The most difficult part is having enough ammo/magic to clear the tentacles with.