I had been cruising through the Necromancer campaign and was surprised to be find that Uriel was as tough as he was. To my embarrassment, I actually had to fight him twice, because, the first time, I focused on the guards and had to give up when he resurrected all of them for the second time.
On my second try, I took out Uriel first and just avoided the guards as much as possible. Even so, I was left with only a small stack of Archliches at the end of the battle.
It's not good game design that the scenario had been such a cakewalk up until that moment, but it is good that the fight with Uriel was so close.
It was the first campaign I played and thus my first boss battle. First attempt was hard as the rest of the map is quite easy and quickly finished and I hadn't built up a large enough army when fighting the to me rather unexpected boss battle. After spending a few weeks building up troops on the second try it was rather easy, so not surprosingly difficulty is dependant on how much troops you bring.
A hint: There is an abyssal worm thing on that map. Save that one for the boss battle and it ties up most of Uriels minions nicely for most part of the earlly battle.
I must have been more efficient on that map than I was on the last of the Haven campaign maps. I found the Haven final boss to be kind of joke, but Uriel to be a real challenge, though I did not have to delay and gather more troops to take him. The fact that he could resurrect all his minions at will definitely was a nice twist.