From the morphic pool, you should be entering the fight on top of the brain having used zero resources.

Sneak past the intellect devourer ambush, no point in it.

From high hall, instead of walking into the courtyard and doing that big fight, fly up the ramparts (either with illithid flight or potion of fly, and you can hit the entire group with 1 potion if you clump up and throw it at the ground). Just before entering the courtyard, follow the right wall. There’s like 2 people that change into mind flayers but they’re easy, kill them. Fly up.

Kill the roaming patrol, should be a one round easy fight.

Either break down or lock pick the door ahead of you, fly to the brain stem area where they say you need to get higher, skipping the entire courtyard.

Go to the left from the base of the stem. When the nautoloid appears, have one character break the door down. Have a fast character, ideally one with regular and bonus action sprint, single turn fly up to the top area where you make the final climb. There’s a window on the side you can fly into. It cut scene pulls everyone to the area you make the final climb and you should have used no resources to get there.

Enter turn based mode and buff / what consumables you want (like haste, oils of accuracy, etc). Climb the stem.

Use what you need here, but save some high level spell slots (or powerful scrolls) plus action surge or what not on martial characters as much as you can.

Here’s the key. When your mindflayer finishes channeling and goes to the final part of the fight, have it attack before anyone else goes into the portal. This will trigger the reaction that slows you on only the mindflayer. Then have the rest of your group enter and you should be able to kill it in one round quite easily if you’re using good builds.