True, the challenge by itself doesn’t specify the requirement of defeating all the opponents, but I think this is the solution for this “issue”. I can affirm that the difference between the players who achieved the challenge and the ones who couldn’t is the fact that the ones who achieved the challenge are the ones who defeated all the enemies.
I think this is the solution for the thread. Despite the fact that you also need to wait for the challenges to be unlocked sequentially. @Hjoerleif