You'll start right in front of the big vine area this time, watch out for the Censor who is behind you. Head forward to that first meat thing that is spitting up funny colored air. You can now jump in this airstream and pull out your levitation ball to float up.
Float up this first and you can land yourself in a big bathtub with a Cobweb. After doing so you can drop through the hole in the bottom of the tub and start making your way up the vines to where we got stuck before. Punch the big piece of meat several times until the airstream comes out then float up it and land on the area in the middle up as high as you can go to get a scene.
Now you need to go over to where the scientist dropped the brain down the shoot and press 'Y' to open it and then walk into it to head down.
Alright, as you've seen the kids brain is now placed in the tank and you need to defeat it.
If you have reached Rank 30 and gotten your invisibility power this fight is going to be worlds easier for you, I have written the strategies below as if you don't have them, but if you do know that you can get out of basically every attack by simply turning invisible and running. It will also help you to get up right next to the tank without him knowing you're there.
The tank has a few different attacks, you'll see an arrow come out of the front of it very similar to the way your levitation aim works. This means that it is targeting you to shoot you, if you walk into the target it will lock on to you and unless you can get behind a pillar before it shoots you there's no way to avoid getting hit. Also note that if you are behind a pillar when it shoots you the pillar will break and be gone for the rest of the fight.
Next thing the tank will do is if you get too far away from it and there is a open path between it and you, a.k.a. no pillars, it will charge at you, this you really want to get out of the way of because it will go through pillars and everything. Just go off to the side though, if you can get moving away right before it charges your best off.
The last thing you are going to want to look out for is when a "?" (question mark) appears on top of the tank. This doesn't mean that the tank is confused, this means that it's going to launch out a Confusion grenade. There is no target for this and similar to the Bomb dude enemies it explodes in a small radius. If you get hit with it your controls will become inverted for a short amount of time and all your abilities will switch and you won't know which one is what, so this is obviously very dangerous!
Now, how to defeat it. There is a spot underneath the tank where you need to shoot it to deal damage. To reveal it you are going to need to get in the position where the tank is going to charge at you. Right before it charges at you you'll notice that it "stands up," this is the time when you are going to need to attack it. The only problem is, you can't be real far away because your Psi-Blast doesn't shoot that far.
So, we need to get up close to him. You can do this by utilizing all the pillars around to basically sneak your way up next to the tank, then wait for it to "stand up" in which case you can shoot it. After you shoot it the tank will flip over and you can run up to it and start punching it to deal some real damage. When you go to punch it though you need to be hitting it kind of on the button, so don't try to punch the front or back but get the side where you can see the button. That's it! You'll need to flip him over and attack him at least 4 times to defeat him.
Be aware that as the fight progresses on he'll become quicker and quicker with shooting confusion grenades, when he's almost dead he'll be basically constantly shooting them so watch you don't accidentally walk into one!
If you ever get in a pinch and need some health and/or ammo you can punch the pillars to break them or just get the tank to break them for you. They mostly all will draw a little of one or the other.
When the tank is defeated it will drop the brain along with blow up all the pillars which leave lots of Mental Health and Raw Energy (well if you had any left and didn't use them all during the fight). However, we have another problem now, the brain is after you!
The brain has two attacks, one is the confusion bombs just as the tank was using and the other is this really annoying green beam that comes out of it and spins around like a radar. If this hits you it will damage you, it's as simple as that. You can only avoid it by jumping over it, or being so far away that it can't reach you, but being that far away isn't going to help you win the fight any. You'll also notice if you watch that the beam will go really fast for a rotation or two and then really slow, the really slow one is going to be the critical time to get in there up close.
The brain moves by jumping around, and it can virtually jump from any one spot in the area to another spot, so there's really no use in trying to run away.
The only way to damage this is with old school style melee action. Your Psi-Blast will bounce right off it so don't even try. You need to avoid the green beam and get in right next to the brain and punch it. I recommend that you use your levitation ball and your ability to float to make getting right up close to him real easy, then when you do start punching! Of course, while you're in here you still need to watch out for the confusion bombs and the green beam. He doesn't take too long to defeat though once you start giving it to him.