that depends on alot of things...
First of all tell use your party and pc along with relative levels. Secondly, do you have the Throne of Bhaal expansion installed also. Thirdly, are you using baldurdash patches or just plain bg2 with official patches.
The first thing i'd do is summon some fodder, they hit hard and fast and even great AC isn't enough to take on that many demon knights at once. They'll cast spells but other than the initial dispells i don't remeber em sling'n anything else my way (that also kinda makes alot of prebuffing a waste of time.. but hey, your call. i'd buff in combat). Another important thing to have is items with negative plane protection, like the mace of disruption and amulet of power, at least one of em will level drain ya on hit, so be aware of that.
Being demons, any type of invis spell is wasted on em (including protection from evil), so if you wanna keep your casters alive keep em outa the room until they get locked on your tanks/fodder. Don't bother casting on them cause they have some pretty heavy magic resistance, if not flat out immunities(i casted the extra level 9 TOB comet spell on em and didn't do a single point of hp dmg... i just stopped try'n after that point).
For an easy victory distract most of em with fodder (i'd abuse a recharged wand of monster summoning) and have your tanks focus on kill'n em one at a time (a fighter/mage with prot from magical weapons would be godly). Iron skin the druids and ranger/clerics, use improved haste as needed and they should fall w/o you loose'n a person. I'd kill the one that appears in the upper right, he's got some damn good armor and one evil sword, not to mention that str belt you're prolly look'n for.
You could just send one char in the room with the boots of speed and just run in order to take em on one at a time, but haven't needed to try it that way.
Fighter/mages can have an easy time of it if you use a time stop scroll that you get from that freed mage in underdark on a simularcum.
Its a difficult fight, but easier than the dragons i think. I've done it with a solo sorc (at TOB xp cap tho...

, they aren't immune to planetar insta kill weapon effect

) and a solo F/M/T (simmy with time stop scroll and improved haste/whirlwind while dual wield'n Celestial Fury and scarlet ninja to... at TOB xp cap also, it was overkill.. but it was damn fun).