My pally friend 1-hit killed her with normal attack, a lightsabre, and fanaticism.
No monster can kill her in one-on-one. (a dozen hell cows can given some times).
I guess the valk gets all the damage a monster would get from a player, so they don't last long. High level Valkyries, CAN kill SOME players with one hit, but I wouldn't rely on it, they walk (slow), and attack slow. I can't really think of any PvP build that would have trouble killing the Valkyrie.
As for PvM... the valkyrie you would have would be unstoppable. Slow, but unstoppable! (I saw mine lasting more than an IK barb - bad player, tho - and she is lvl 18!)