If you want to get a good score, you will have to manually play most bosses in the game for certain mechanics. In the case of Toxic, it’s the Flare skill you need to be wary of.
This skill will kill your units, so you need damage immunity to survive or damage distribution plus a healer that can keep up. For this reason, there are two units currently that are “mandatory” for getting a high score in this boss.
Both characters provide damage immunity with their Blossom skill, so make sure you turn off auto only for them (you only need to use one, not both in the same team). You also have to time when to use the skill to have enough time to apply said immunity. Wait until the bar is almost done (around 5-10%) to proc your skill.
This is the setup most of my guildmates run for the highest possible damage output but requires RNG and manual play.
I run a more auto-friendly composition since I’m lazy with Rayhou and Itus / Iliana for almost permanent shield uptime lol.