So, my assassin character in Requiem just one-shot one of the most powerful characters in the game.
See this bastard:

This is Lokil. In ordinary, unmodded Skyrim, Lokil is about as difficult as any other vampire to kill, which is to say, not difficult at all. A couple of hits from a smithed silver blade will take him down, no problems.
Requiem, of course, changes this. Because the Dawnguard questline, for whom Lokil is a starting boss, is considered high level quest material, Lokil is instead a level 80 battlemage decked out in enchanted ebony armour, meaning he looks more like this:

Because this is Requiem, this adds in additional problems. Ebony armour has huge bonuses to defending against arrows. Undead are immune to poison, so put them daggers away. As a battlemage, he has a massive magicka pool, so he's mostly immune to illusion too. His armour has fire resist enchants, one of the key weaknesses of a vampire. Up close he wields an ebony mace (in other words, he will tear through your armour) and he casts Frost Cloak at the start of combat, so say goodbye to your stamina, and your ability to move. Restoration can still take him out, dual casting sunfire at him does have an appropriate effect, but eh, restoration. Lokil is a notorious check on character progress, and
as you can see from this video, even skilled barbarians have problems taking him out.
Assassin type characters, especially assassin archers, are generally considered the weakest build in the late game.
Turns out though if you shoot him in the head with three exploding dragonbone arrows of fire, from a fully tempered ebony bow, with full perks in sneak and archery, he dies pretty hard. As does everything else around him, since these are basically fireballs attached to an arrow. In the face. Repeatedly.
I can only conclude everyone who played an assassin before me is retarded, as I'm kicking ass in the late game.