Ureus has a fatal weakness; he's walled up in his own little enclosure. You are oviously meant to knock down said wall with an Aard sign and then engage him in an epic melee. Instead I thought "Wonder if an Igni blast will go through the wall without blasting it?" It didn't, and I just stood there and burned the critter alive (well undead). It took awhile, but nowhere near 5 minutes. And there never was any doubt to the outcome of this particular fight. The Dagon fight is tougher until you realize the way to kill him is to kill his surrounding fighters. Every time one of them goes down Dagon gets weaker.