Swimsanity!

I did this in 4 player coop and opposed to the solo-method, the game crushed only one or two time(s) in our countless attempts. (I tried this level several times in Solo,too and the game crushed between 3. and 4. turtle every time)

We chosed Shock, swimming tank, slow-motion & harpoon as setup.

A hint for the 2. turtle.
Activate your swimming tank or mooba-suit immediately, when you enter the boss area and shoot constantly on the turtle.
After a short time of spamming shots, the turtle is stunned and it starts the same dashing-routine / pattern again (the turtle starts on top, middle, bottom).
Attention: When the turtle reaches a third of its health, the pattern reverse to: bottom, middle, top or bottom top,top)

A hint for the 3. Turtle:
Stay on the bottom left corner of the Boss area.
This a safe spot, where the turtle cannot hurt you.

Before downvoting, please tell me your reason in the comments .
hope this helps

This is one of the more frustrating achievements I've come across. Not because of difficulty, though it is tricky, but because the game crashed toward the end of the level every single time. I'll get into that.

My first tip is to keep your shield and health full. That may seem tough to do since the power-ups are "random," but they aren't random. They randomly cycle through three set power-ups. You can see the power-up behind the bubble that you have to shoot. Aim, and when you see a shield or health, shoot quickly. It's not as hard as it sounds. Also grab them when enemies drop them.

Do not be afraid to use your unleash. I used harpoon because it helps kill the turtle boss. There are so many enemies on this difficulty, you'll refill your unleash in no time.

Always prioritize the seahorses with the bombs. They can insta-kill you. After that, kill anything shooting at you. The green guys (jellyfish?) that shoot were my next priority.

This game shifts between a set number of enemies and infinitely spawning enemies. If you learn where which sets do which, you can adjust accordingly, though for the most part, I just went fast.

For the turtle boss:

Phase 1 - Pretty easy, ignore the portals, just stay out of his way when he laughs. He will lunge multiple times before he "resets," but he isn't moving too fast at this point. If you die here, you should restart and try again.

Phase 2 - Much more annoying. On normal, you can teleport to safety, but that's trickier with the barriers in hard mode. I stayed close to the portals. When he would lunge, if you're in his path, get in the portal to safety. The next few lunges before he resets come quickly. I would gently tap the stick toward the portal repeatedly, which made it easier for me to get into the portal quickly. Remember to keep shooting, and grab power-ups when you can. Use your unleash. The quicker he dies, the better.

Phase 3 - Very similar to phase 2. Stay close to the portals. Tap toward them, and tap in once he lunges in your path. You need to be quicker on this one than phase 2, since there are multiple portals to race to.

HERE IS WHERE THE GAME KEPT CRASHING AND MADE ME WANT TO BREAK STUFF, SEE BELOW.

Phase 4 - If you managed to get through the crashing, this one is actually a bit easier than phase 2 and 3, in my opinion. Try to avoid shooting the laser capsule thing once your firing path is clear. The reason it's a bit easier is because the turtle is moving so fast, that he nearly always (if not always) goes top to bottom to top to bottom when lunging. There doesn't seem to be any middle ground. So after he lunges, if you're at the top, portal to the bottom, and vice versa.

FOR THE CRASHING:

I've read if you have an X|S this may not be a problem, but the game crashed every single time for me between turtle 3 and turtle 4, when the big purple fish ships would come on the screen. I tried a lot of methods to stop the crashing for hours, and here is how I finally got through. It worked twice.

Stay at the bottom of the screen. Move quickly. Do not shoot. Do not shoot? Yep, do not shoot. You're going to use teleport (A button) whenever you're in trouble to keep moving left. Do not spam teleport or you'll run out of them very quickly. I used my unleash (harpoon) about a third of the way through as well both times. I think the key is not shooting. I spent hours and hours trying to keep the game from crashing, and this was not only the one thing that worked, it worked twice. Hopefully, this works for you, as well.