You'll find yourself in some sort of airbubble underneath the water. There's not much of anything you can do down here, but if you head left you'll see a bright glowing wall that you can punch through, behind it are some stairs that go only one way and that way is up, at the top is a big bell and a rope hanging down, jump on the rope to ring the bell and bring out the Lungfish.
In this fight you are stuck in this air bubble area and the Lungfish is right on the edge sucking in items and also spitting out items. That's really all he does, if you get too close to him you'll get sucked in and if you're too close when he's not sucking he'll spit something out on top of you. It's pretty obvious how to avoid both of these, but hurting him might not be quite as obvious.
If you attempt to shoot him you'll notice that the shots seem to kind of hit the edge of the bubble and bounce off and it doesn't do much good to throw (via Telekinesis) items at him because he's already sucking them in. However since he's already sucking things in, why don't we alter those items, to further explain, you'll notice that the boxes he is sucking in appear to have a tac symbol on them, so hang out decently close to him, not so close that you'll get sucked in, and when the boxes come by punch them open to break out the tacs and send them flying in his mouth to do some damage.
After doing a bit of damage things will change up and the camera will become the monster's eyes. He'll now be "chasing" you, although not super fast, but you'll need to run away from while staying inside the bubble (so the water's hands don't get you) and you'll need to platform your way across many obstacles. There's not really a lot of ways to go, you just have to keep moving, so I'm not going to write out exactly where to go as he chases you. Just keep your eyes out for the glowing walls you can break through, ledges to jump on and poles to swing on. After a bit of chase the fight will go back to the fish sucking up things.
Every time he switches modes you may (or may not) notice that the bubble gets a bit smaller so you have less room to move around. Be aware of where you are so that you don't walk out into the water and get grabbed by its hands.
After the second chase he'll come out and go after you by foot. In this area you'll notice that there area a lot of clam shells on the ground and that as Lungfish chases you around he swings at you with the little antenna type thing on his head. If you have gotten too close to one of these clams at any time you may remember that they snap down on you, well what you want to do is to stand behind them so that when Lungfish comes over and swings at you, you can jump out of the way and his antenna will get stuck in the clam. At this point you'll be able to pull off a few melee hits and then repeat. You'll have to get him stuck 3 times to kill him.
When it's defeated Ford pops out of your ear to share some thoughts with you, after he goes back in go into your backpack and get out the Psycho-Portal you grabbed from Sasha's lab and use it on the Lungfish to get inside his mind.