Be sure to have Gumbo (the grey angler-fish) along when fighting him. Type-U(Ulysses the cyclops-golem-thing): Kill him quick!!! His energy spheres don't just kill the fish that they're targeted at, but anything that they touch. If he is near any of your fish while blue, shoot him anyway, just to move him. Type-P(I don't know the name, but he's the octopus): Shoot him while he is red, shooting while blue heals him. Type-D(Destructor the robot): Shoot his missiles before they hit your fish, then shoot him. Type-G(Gus the giant fish-baby-thing): Feed him until he explodes.
I would have posted another spoiler here but once for now is enough :) this other spoiler is for how to get thousands of shells but not having to click on anything AT ALL!!!!!!! i'll post it another time so keep checking back here if you want it ) now im just collecting shells to spend on things to put in my own personalized virtual tank ) I love this game!! i finished it three times already cuz i wanted to unlock some of the secret characters :P i've unlocked 2/4 so far. to prevent him from getting closer to your fishes, just shoot in front of him once or twice so that he'll turn around and go to the opposite direction from your fish. oh and while he is blue, he sometimes tries to slowly come closer to your fishes and immediately eat them as soon as he turns back to red. you can monitor his health if you have the white beluga whale named Blip in your tank at the same time. it says that at the bottom of the game screen when the psycho octopus turns blue. if you shoot him while he is blue, he gains back his health. shoot him as many times as you can while he is red and dont shoot him while he is blue.