The answer arturgaldino gave is bang on correct, just with one addendum. The Elf Overseers will ONLY respawn once after Thornfire is reset. So the fires have to be put out and Thornfire stopped before they will spawn. Then once they are killed they will not respawn until Thornfire is put out again. thornfire requires the Overseers to be killed AND all the elves in Shadowthorn to also be slaughtered. So the whole Mini World Change must be played out before they spawn again.
So the best time to kill them is right after SS on the day Thornfire resets to the Guides by the ships responses to asking about Thornfire - Countless Firestarters are in their cells below Shadowthorn, but right now they are safely guarded.