If you can't beat him because you're running out of time then there isn't much I can suggest to you to help. Is this the 2nd tiem you fight him when he's underwater on garaden? Or is it the first time? The 2nd version of him is probably the most aggrovating boss in the game. I had to extend my lifebar by collecting gems in order to beat him.

Have you figured out where to hit him? If you already know where to hit him to do damage, you'll just have to figure out a way to do it faster.