Synopsis, multi-stage fight:

Stage 1: They enter the brine pool chamber, find a sturdy dome protected by a Null Psionics (antimagic) field. Six smaller chambers around, each with an immense lever meant to be pulled by a huge tentacle. Each room is filled with a theme or object important to the Elder Brain, and some minions. They fight off the minions as a warmup and to spend some of their resources.

Stage 2: All six levers are pulled, shutting off the antimagic field and making the brine pool rise from the floor. It's enclosed in a tank of sturdy glass-steel but has six ports, one for each of the Elder Brain's tentacles. The tentacles emerge and the fight begins. It hits them with a powerful Mind Blast to try to stun as many as it can.

Stage 3: But wait! Each character that is stunned, instead of being knocked out of the fight, is instead Astrally Projected onto the astral plane, where the Elder Brain is also residing, but without its tank! They can attack it directly, but their bodies are stunned on the material plane, where it can still grab them with its tentacles and try to extract their brains - intent on filling their skulls with Voidmind slime to make them fight for it instead! The party wizards take to using Grease on stunned allies to increase their CMD and this literally saves one of the PCs from being brain extracted at the very last moment.

Stage 4: They cut off or petrify all six tentacles and voluntarily fail their saves against Mind Blast to re-enter the astral plane and try to finally kill it. They get it to single digit hit points and it tries to teleport away in the Astral - only to find that the Sea Elves, whom they had asked to help them ensure this thing didn't escape, had surrounded it with Astral Seaweed that trapped it close to them, and they landed an extremely satisfying killing blow.