While getting killjoys isn’t particularly difficult (they occur naturally), the conditions needed to earn them are completely backwards to what challenges should be. Essentially the game is asking you to kill the guy who’s been terrorizing your team. In other words a killjoy is only rewarded when you or your team has been playing badly. It’s like telling players to get Comeback Kills (get a kill after dying 2 or more times in a row). It’s only given to player that are performing poorly. Ironically Comeback kills could be forced and don’t rely on the performance of the other team.
tl;dr This is an objectively bad challenge that, at best, isn’t challenging since killjoys can’t actively be chased, and at worst, encourages game throwing.
That said, the general improvements to the Tenrai Event have been great so far. Hopefully future events follow its path.