Part of the problem with these "make the fight harder" events is that the rewards have thus far been rather subpar. Do each fight once for primogems, but after that you're pretty much just getting mora and little else unless you're a collector that just has to have the profile background. The events with shops tend to give you a much better range (and quantity) of rewards, making it more worth your while to play through the event repeatedly and find ways to be more efficient. Instead, it's "increase difficulty for more points" for little reward and "use these characters for more damage", except I don't have those characters so that's a great help.