I'm able to finish off the super boss of the second phase with two tea sets; usually it dies during the first round of battle against whichever helper squad I brought in. My fourth squad tends to deal the most damage overall due to how it's set up (Iberis + Sakura + Flowering Peach + Mei Plumblossom + Kale: 1.6x skill proc rate for everyone, +78% 'standard' attack power for everyone, +35% additional attack power for everyone since it's a boss fight, boss takes 105% normal damage from attacks, four of the five members have a single-target special attack).

It helps that Mei and Kale are the same type as the pest so they're basically dealing critical hits with every attack.