I've now defeated the light element Orb Boss III level 3. The heroic seraph squad that tackled this challenge was swimsuit Tama, tuxedo Tsukasa, light Megumi, swimsuit Yuina (with the Saintly Smite ability introjected from base Yuina), base Erika and ninja Mari.
Erika, Mari and Yuina were the same memorias who defeated the dark Orb boss, but Tama, Tsukasa and Megumi were exclusive light warriors recruited for this mission alone. And swimsuit Yuina is only good in dark and light fights, so she was basically an exclusive character for this mission too.
Only by equipping all six of my seraphs with damage down bracelets was I capable of surviving this boss' attacks, so it took a long time farming the elemental resistance bracelets before I could undertake the real challenge. But I beat the boss on the first try once those bracelets were equipped.
Now all that's left is the ice Orb Boss III level 3. I don't have any decent ice memorias so I think I'll just ignore this challenge for now. I'm just happy my dark and light memorias saw some actual use in a high-end battle -- I didn't handle everything with the same 'power squad' but actually branched out a bit and got creative. I also beat the boss at the rating I was supposed to beat it at -- the recommended rating was 19,000, and my memorias clocked in at 19,113. I didn't brute force my way through this boss, I used the proper tactics and won fairly.