That's what my Mechromancer wants! Smash the state (and bandit heads)!
For reasons that I don't recall, Lord Godfrey (having been risen and put to work in the Forsaken assault on Gilneas) turns on Syvlanas and shoots her dead. She is promptly resurrected by the Val'kyr, no worries, unlife goes on. If Sylvanas doesn't seem any worse for wear after her reanimation, couldn't she just keep doing this? What would it take to kill her once and for all?
There were 9 val'kyr who remained after Arthas' death and went to work for Sylvanas. One died to seal the bargain between the vrykul and Sylvanas, one died fighting, and three died to bring her back after Godfrey killed her, leaving four val'kyr. That's maybe one more resurrection if the formula remains the same as the first time. So I guess the answer is "kill her two more times"!