On the slayer skill guide under challenges, the guide says you can die before wave 31 and still have enough monsters left on your task to give it another run. However, I personally failed on that task on jad himself and checked my monsters left, and it was 9 more. On top of this, I got the task again, got to wave 5, then realized I forgot some items, so I left the cave and went back in. On wave 60 I completed my task, and did not get the bonus for killing jad. There are only about 8 extra monsters, if you leave any time after wave 5 you will not be able to get the bonus for killing jad.