As for the crazy high damage, I'd have to investigate but my guess is that in some cases you're seeing the damage being done to your barrier health. Barrier math is calculated before armor and other resistances so the numbers will keep getting higher the higher the creature you're fighting is.
No, this is done even on guard/health. I can provide video on request. I can isolate a venatori marksman, killing everything else around it, but not damaging him. Then damage him, with his actual % health threshold being unknown triggering his attack, and then he'll perform this rolling draw to my face.
Edit: Nevermind, Heroic Mass already provided said video as I was typing.
I personally don't mind, as this makes the archers lethal, and taking it away would be taking their teeth away. However, I do recognize it is another barrier to entry making melee characters far even harder than ranged.
This move is the problem. They turn their back on their target, sprint, and then fire for 1k+ damage. I'm guessing this is a bug, and not intended, as there is little to no way to avoid it.
There are ways. The first of which, not attacking them until you're ready to kill them. Second category is fast offensive incapacitates like pommel strike / charge (can you tell I was just playing Katari), third being avoidance / invincibility skills (fade cloak / combat roll / fade step ) with the right timing. Lastly, fighting near and using cover.