Spoiler!
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I can't seem to beat the elder orb and beholder in the underdark. They always waste my weaker party members, or petrify them, or charm them or something. I've got to get the blood for the matron mother, but i've already got the blood from the master brain. I just want to be able to beat the elder orb and the beholder. Also, I killed everything except for the prince in the other section, but everytime i get him past barely injured, he just gets better, and wastes me. How do i kill him?

i neva actually killed them, i ran in got the ******* hilt (wont spoil it) and pegged it. try spells like minor spell deflection, spell turning and if i am not mistaken, there r robes that do that as well.
anything that will hasten ur party (haste, improved haste, boots of speed) will help and also buff them up like normal with chant, bless etc.
i dont know but chaotic thoughts might help.
i usually avoid beholders if possible cos he petrifies chars including 1/3 pc's which means u never get them back, so i summon and buff a tank and let him do it all. (hey, he is level 22!)

You have to put the diseased tad poles in with the healthly ones to weaken the Kuo-Toan Prince.

The Beholder Lair. I have good experience with Skeleton Warriors. For some unknown reason, even the Elder Orb has always problems with them even if it can kill, petrify or imprison them.
The Kuo Toa Prince. Improved Haste plus good weapons and good weapon proficiences (++ at least) plus concentrating all your forces only on the prince should do. The last two ingerdients seem to be essential. I've never had problems with him even without Improved Haste or using the spoilt toadpoles.

The only problem when it comes to fights in the Underdark is the Prince (that I had). He kept regenerating. I mowed everyone down though.