Personally I like to go into the main room with the illithid and push the 3 gobbies closes to the pit in with spiders. That gets rid of the wizard and priest then I wipe out the rest of the room. You can also go into the pit room first and clear those 2 sets of 3 out without setting up the entire fight, but you have to watch out for one gobbie that tries to run and hit the drum. If you get those 6 out of the way first, usually the priestess and her room don't join the main battle.
