This thread is dead, but should anybody ever want to look it up for later references, I'm sure posting strategies wouldn't be bad.
I carried the "anti-typical" party - PC (Priest of Helm/Thief), Imoen, Nalia, Anomen, Sarevok, Viconia - and this worked well.
Crowding into a corner of the room or in another room all together (with you AI off) you can usually lure one or two of the opponents into your ambush. If you're being bombarded by spells and getting killed through that, a few Skeleton Warriors with thier magic resistances can affor to take it. I had three clerics in my party, so summoning undead wasn't much for me.
High magic resistance will make lower level spells practically useless, so I had my mages hit the baddies with Lower Resistance spells and two of my priests use the generic diabling spells - Miscast Magic, Hold person, Chaotic Commands, ect. The front liners went after enemy mages as they are the ones who pose the real threat.
As for the Merelith, she stinks. When you get to the beast, hit her with every dispelling spell you've got. She's immune to normal weapons, and likes to cast Protection from Magical Weapons all the time. Have three or four characters wither down her HP while your mages keep her in check.
Typical Beholder. It's a Hive Mother, isn't it? She likes Flesh to Stone for some reason - she once took out four party members of mine before I took her down. Be prepared for that.
But that's just how I did it.
Those are just the things I remember.