Swimsanity!

There is no guaranteed tactic to win this I'm afraid, you're going to have to practice the early parts and try to have 3 hearts when the final boss phase begins. The unleash you should use is either Aqua Tank or Mooba Mech. The Mooba Mech unlocks at 150 gold orbs. You get the most gold orbs for passing a level for the first time on either normal or hard difficulty. The mech or the tank will basically gives you an extra life at the boss, unless you take a physical hit from the boss, you will die and lose a heart.

You should stay as far as you can from the boss to the right of the screen, this is mainly because if the blue shock enemies hit you, you'll likely get hit by the boss or the boss will launch a bomb to your face. So what I did when the battle began, I escaped to the right, activated the mech and started damaging the boss while quickly dispatching those jellyfish enemies. There are "checkpoints" that replenish your health, the level is always the same, only the enemies spawn randomly. So you should learn when those come around and try to stay alive with the current life until then. Yeah, the game gives you loads of crap, I know. It is just part of playing Swimsanity. Sometimes you may make it with luck, sometimes not.

The boss shoots a giant laser beam to the middle of the screen once its health is halved. I always stayed at the bottom of the screen, because I found that bullet cannon to be more dangerous than the bombs. When the boss is near dying, it will stop moving and aims the laser beam in 3 directions: up, middle and down. You need to be careful here and fast in your reactions. You basically have to keep and eye out for the cannon and use to teleport immediately if it aims at your location. So if it aims at the bottom of the screen, you'll have to teleport up quickly. The health bar is painfully long, but it should die eventually and you are one achievement richer.