[Neomi]

It doesn't sound like his party is high enough to summon planetars/devas or to have that many HLA's if it's a party of six having difficulty with the demon knights (although I agree that a planetar/deva would be a great summon to try if you've got one).

Negative plane protection is key. Also, try setting up some type of defensive position protecting Aerie and Imoen, even Anomen, so that they can cast without being disrupted. You, Minsc, Keldorn, and a few skeletons should be enough to hold off the demon knights long enough to get in a few holy smites.