OK, just to add the final words hehe..
You DO NOT need to turn into Slayer to open the doors. There are 2 ways, as the doors tell you: "either a person of godly might or a total brainiack can open the door". Which means that either you turn into the Slayer OR you charm a Mind Flayer. If REP is something you want to keep, then forget the Slayer form and simply use the collars to charm the Mind Flayer. Once you open the door, the flayer will go hostile, you can either kill it or re-charm. If you kill it, then no worries, there will be some further in. Once you beat the brain up, ALL doors will be open for good. Just keep the fluid you get from the brain, it can become handy. As for charming the brain for easier fight, well, why? Golems are goodie extra XP, let it summon them I say
Pay attention though that the Illithids and Uliathid (or whatever that thing was) do NOT open the doors! It MUST be Mind Flayer!