Nikke - Union Raid (UR7): Chatterbox [Stage 3] 194.5M+ damage
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The units I used for this battle are Dorothy, Centi, Modernia, Privaty, and Rapunzel. The team's Skill levels and equipment are shown before the fight begins in this video. I recommend using team for ensuring survivability, the fact that this boss is weak against water element units, Centi's Skill 2 helps prevents damage, and Modernia is very useful when it comes to taking down the QTE/Yellow & Red circles sequence. Be very wary over Dorothy's Burst I's aiming for it can miss, and when Chatterbox charges his fist in flames, get ready to block when he leaps for the impact when he lands can be very damaging towards you team. 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/