Use Your Abilities, But Watch His
First and foremost, it would be very wise to summon in as many elementals and planetars as you can (5 total for those who do not know). Balthazar has tons of special abilities, he can stun, he has an effect that is basically a sunfire (as per the level 5 mage spell) centered upon him, so keep your casters away from him. If you have the ability of greater whirlwind for your tanks, use it! Balthazar relies on his special abilities to really be a tough opponent, as he does things such as shadowless kick, which basically is like a dragons wing buffet, only he can do it to only one character. Balthazars magic resistance is high, and rarely would any of my spells land on him. He has a huge amount of hit points (280 I believe), and regenerates.
As stated earlier, hit him hard and hit him fast so that he does not have much time to get off his special abilities. Do not worry much about the other monks, I tended to let my summoned creatures distract them so that I could focus my primary tanks upon Balthazar. Keep your tanks healed and ready, and have your mages casting time stops so that they can ready their defenses(as I did in my fight with him) or summon more creatures to aid you. You will want characters with bad saving throws as far away from Balthazar as possible, as he can and will use his special abilities without remorse on you(as I found out the hard way). Be sure your fighters have plently of heal potions readied, once you defeat Balthazar you will be brought to your pocket plane, so, unless you want extra experience for killing his monk guards, just focus your efforts on him. Just remember, hit him fast and hit him hard and you may surprise yourself with a quick kill.
Submitted By: Ac Playa
Improved Alacrity and Time Stop
Alright, defeating Balthazar is really easy, just make the right preparations after you've entered the monastary (with anti-fire spells and a Spell Sequencer with 3 Lower Resistense spells in it). Then, after you've spoken with Balthazar, launch the Spell Sequencer on him and let you're best spellcaster with Vecna's Robe cast Improved Alacrity and Time Stop while Balthazar is busy fighting your warriors. Cast loads of Abi-Dalzim's Horrid Wilting and don't forget the Dragon's Breath.
Submitted By: Robert de Koning