Yea, the harping on about R's is rather random since the original comment that kicked this off specified SSRs

But as something to think about it for fun...
Given sufficient base attack and assault, this ragnarok may be doable with just R's by the 3rd time this event rolls around. (gasp, 2nd reprints are a thing? Apparently so as the latest event on DMM that just ended today is Ixion for the 3rd time). I'm thinking of Fire in particular. Why? Konohana Sakuya. She's an R with a Sniper Shot on 10 turn cooldown, plus a 500 HP party heal every 5 turns. 10 turns is a long time, but if you're experienced enough to accrue the atk and assault necessary for this, you'll play fast enough.
10% A-frame debuffs from Baphomet and Nergal
For that 5th slot... I guess the Fire variant of Mephistopheles and try to land the 2nd skill (backup B-frame debuffs, plus the fire resist debuff would be quite useful). 1st skill is a pile of DoTs+blind+dizzy. The time spent on processing all the DoTs may or may not be worth it?
You can run either Cassio or Andromeda with Black Propaganda, depending on how you weigh the extra 400 HP per 5 turns vs Chaos Magic.
It still won't be easy, but it wouldn't surprise me if in the future, some DMM player pulls it off.

Water.... maaaybe? I'm not confident in water here because I don't see any of its R's having atk/def debuffs. Still, you have tools to play with.
Sequana - single target 350/500 HP heal + 1000 damage barrier every 3 turns? Is that right? Single target debuff removal + debuff block on 5 turn cooldown
Ganga/Ganges - party-wide debuff remover + 2 turn debuff resist buff every 8 turns. Oh, and a single target damage skill, whatever.
Dagon - -15%/-25% rage meter every 8 turns. Attempt to reduce enemy charge by 1 every 6 turns.
Apsu - party-wide regen for total of 750 HP every 6 turns. And a single target damage skill, whatever.
Kushinada with her single target 800 HP heal every 5 turns is an option to replace Ganga or Dagon, depending on your exact strategy.
I'm thinking Cassio with Sniper Shot plus Jormungandr buddy.