?
You probably ignored the mechanics. There is a single battle where you need to kill adds, the first one after meeting your allies. You clean up the adds, then go inside the castle and your companions straight up tells you that you need to rush to the top door because the enemies will keep flooding in. Then the encounter with the nether brain is the same, you need to kill the mindlayers by the brain, the dragon and that's it, you're supposed to use the netherstone on the brain to channel for 2 turns and open the portal, then kill the brain inside.
If it took you 4 hours, you probably ignored what the game was telling you, or are running an underleveled or really badly assembled party comp.
4 hours? How?
