Several ways. Keep most of your party out of the area and send in a bunch of summoned creatures. You should be able to deal with just the lower 2 knights before you have to worry about the upper ones.
Also, take a look at Simulacrum in the strategy section and use the Protection from Magic scrolls on a couple fighters if you have them.
You could also try making the sacrifice, clearing out of the area, then throwing Cloud Kill into the area from out of sight range. If they're immune to that, try Ice Storm or Death Fog. |