The Gatekeeper is dead easy if you do it the right way, spoil:
with the special bone arrows and his mom's tears to poison them end spoil.
Xivilai, imo, are one of the easiest Dremora to kill. Dammit, they have no armour!
Minotaurs can be tough. But poison on your blade works wonders...
I only tried to kill Umbra once, but never again because I felt sorry for her. But she was really tough. Whupped my char hollow. Mind, I was new to Oblivion at the time.
Wisps are a real pain in the butt until you have a good magic weapon. After that, no trouble at all. Though it is wise to have potions or spells to restore attributes...
In Shivering Isles - which I have not finished yet - the Knights of Order are tough; maily because they just keep coming. But once you know who to kill first, and what to do next, they are not QUITE so bad. Mainly because you get to the end of them!
Mainly, you need to know your enemy. Second you need magic weapons. Third, a spot of poison on your blade works wonders!
