Thread: [Event 37] Crucible of Combat vs the Demon Sloth

Given current info, I'd rank Union buff priority as follows:
ATK = Affliction RST > Double attack > Triple attack

That's based on auto-battle; I don't see why you'd ever want to manual battle Union events. Affliction resist keeps you alive longer, and the others increase damage, all with the goal of maximizing damage dealt before the DoTs kill you.