Union events might seem like team-building exercises... but they're not. Strong players can auto-battle solo the highest level Demons you'll need to kill to get full rewards.

As for anti-Overdrive builds, both Water (Cthulhu + Snow Raphael) and Wind (Gaia) have effective counters for Overdrives. Other elements (and also these same 2) can run Burst builds to deplete the Rage meter before Rage Overdrives can go off. Anti-Overdrive builds are particularly effective in Accessory Quests, where the final boss's Rage Overdrive is extremely dangerous.