I've always found necromancers a pain in the butt to kill, they're not hard to kill, just hard to catch. Always running away from melee.
Otherwise, Goblin Warlords and Ogres are annoying. I should go back to the SI, see how brutal their monsters are.
Necromancers are difficult to kill but they are people, not "monsters"
That's why I mentioned Goblin Shaman. They are monsters, and are really annoying like any enemy that summons runs away and heals themself. Maybe not most deadly but they sure are annoying.
...Even the fabled Mannimarco was woefully underpowered.
Mods can fix that. With the OOO mod he's extremely deadly.