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I have tried twice and failed both times. Seems like one of those battles where you get lucky, or you don't. I suppose if I read all their stats I could devise a plan to exploit weaknesses. I wonder, though, about being able to read everything about each enemy. It would be like running a (paper and pen) adventure and giving the players access to monster manuals whenever they run into a problem.

Well, I did, but it took me a lot of retries. I think you're pretty much right about having to "get lucky" to a fair degree.

Although, part of my problem was that I was initially trying to win while keeping the two NPCs intact. But the commander guy just runs off where I can't help him, so I eventually gave up on that (I kept the wizard alive, though). Has anyone beat the final fight with both of them alive?

I've seen at least a few people say they managed to do it. Here's the first person I remember doing it. He explains how he did it.

I've managed to finish with that battle with both the Commander and the wizard surviving. It took SEVERAL attempts for me to manage.

I beat it with the typical 2 fighters/wizard/cleric team, couldn't keep any of the allies alive though. Took a lot of reloads, but basically, the key to victory is to dominate spam until you get lucky. The Balor and/or Vrock, are the best to bring on your side (at least they were in my case). What also worked for me, was pulling back to the bottom right side of castle (in the corridor) after a while, to heal and buff up.