It has to be Tyrande. Genn would be the best, but he is not here. So whoever kills Sylvanas should be Tyrande. But I wouldn't mind if the Jailer kills her.
*waves to Anduin* Leave us!
Sylvanas: No, you stay here! I'm in charge.
Jailer: Do you feel in charge?
Sylvanas: I gave you lots of souls?
Jailer: And that gives you power over me?