The Bombshells Wonder Woman was easier to get than I thought. (I played through two sets of Arena matches, with one of them as a random character, with a lot of the experience coming from PvP or rifts)
I don't know if this makes a difference (since I'm not sure if Rifts will be available after the shutdown), but I'm almost done with the Fighters Road - I need about 1500 experience to get the last character (Marvin the Martian), so as long as I get to the end of the battlepass (or play enough Rogue Rifts), I'll get it.
It's a shame that this game wasn't given more of a chance to survive, though I suppose two separate launches and a heavy focus on monetization screwed it over. (on a related note, I'm surprised that Gleamium isn't more widely available as a result of the shutdown - there are a ton of costumes that are seemingly impossible to get now because there's basically no way to earn Gleamium)
EDIT: I got Marvin the Martian, so I'm all done the Fighters Road.