I soloed a LV52 Ultimate just now outside of Burst time with full Union buffs (attack, HP, double attack, healing) and a Water team. It came down to a combination of some debuffs landing, lots of healing (Andromeda and Sol), and having Burst saved at opportune moments to avoid the Rage Overdrive. The double attack buff was likely essential to having Burst in time for each Rage Overdrive. But for auto-battle, replacing double attack with affliction resist down and going Mordred might have better results.