The trick is, you must fight them one by one. Of course 5 at the same time would be a force to reckon with, but when there is only one of them... heh, heh, heh... ...
You will note that they do not really spawn near each other. So here is what I do.
First, send somebody in the room so that you can see the Altar. Then move everyone out of the room. Summon some weak expendable creature and click the Altar's location. When the Demon Knights spawn, you then start summoning your creatures.
When you have five of them, send them just beyond the door, 2 Knights will engage them, but the rest will not, after the 2 have been deafeated, a third and possibily fourth may join in now. Summon more as necessary, alhough the initial five should be okay, unless the Demon Knights get too many critical hits and the summons made too many critical misses.
Skeleton Warriors rocks against them because the Demon Knights waste a lot of time
casting spells at them and we all know how effective magic is against Skeleton Warriors.
By the time they engage in melee, the Skeleton Warriors have one free round already.
Furthermore, when you have 5 Skeleton Warriors wailing on them at the same time, it won't be long before they croak and die.
At an average of 15 damage per hit, that's 75 damage per round... ... and they rarely miss.
Oh, and only in hard and insane will the enemy deal more damage if I recall correctly. Are you playing in such difficulties?
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If not a Sorcerer casting total oblivion, then a Paladin meting out justice. Otherwise, I think I will just be a bard and torture my enemies with, "My Heart Will Go On".
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Baldur's Gate Item Mart