Wow. This game really is hard. I just managed to kill spectral commander in spiral cave. It took me every dirty trick I know and pretty much all my scrolls to take him out. First I lured his retinue out and took them one by one. Then I used every boost item in my possession and went back in and just managed to kill him with concentrated attacks. One of my characters even managed to get there out alive to use resurrection scroll. Then I took out his summons one by one. The final insult was the undead posse waiting near the exit. If I had had one less resurrection scroll I would have had to do that again.
I'm playing as normal and I had level 10 party. 2 dual wielding fighters, a priest and a sorceress. As far as I can tell my build is not totally suboptimal. All I can say the difficulty can be really brutal sometimes.