Nikke - Union Raid (UR10): Dual Ring [Stage 4] 238.7M+ damage
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The units I used for this battle are Frima, Helm (Aquamarine), Snow White, Suger, and Maxwell. The team's Skill levels and equipment are shown before the fight begins in this video. I recommend using team not just only for survivability and fact that it's weak against iron, but to ensure saving some of your best units for the other tougher bosses. Just be sure to block before this boss does its 3 seconds charged attack, otherwise your team will be stunned for a few seconds. Best of luck to you.
To find out which units are in the meta, I recommend checking out this tier list so that you know who to focus on when it comes to skill building priorities:
https://www.prydwen.gg/nikke/tier-list/