This battle took me a week to luck out on in my stage 8, no upgrades, no shrouds game. With upgrades, the battle is a lot easier but still fairly difficult (the ending is where most failures happen). Trying to go for speed is kinda insane. But, I'm kinda insane, so together we're kinda sane. ^_^
This is about as fast as I can do it. Poison of course does the most damage at 52,800 HP/sec, but a single chain of Ruins (+ Faith + Deshell) does over 70,000 damage so Offensive Screen is still pretty good.
What needs to happen is for the first stage ("stage" = "level of strength of Vercingetorix") of the battle to consist of only two rounds ("round" = "period between Impenetrable Auras") and the the fourth stage to consist of only one round. The former happens if Poison sticks immediately in both rounds, Deshell sticks quickly in the second round, and you immediately use Offensive Screen after Poison (both rounds) and Deshell (only the second round). The latter requires quite a bit more luck: Poison must stick almost immediately, Slow must stick fairly quickly (which requires Deprotect and Deshell to stick very quickly), Fang and Light both need to keep Haste and Faith (i.e., cannot be hit with Putrescences, let alone survive overall), Wicked Whirl must not kill Snow (otherwise Putrescence on Light/Fang is likely), Wicked Whirl must not be the first move (otherwise you probably can't stick Poison immediately, making a kill in the first round impossible), and not too many Wind Shears happen (besides Wicked Whirl, the only attack capable of hitting Lightning and maybe Fang). That's quite a laundry list.
![](https://s2.save4k.ru/pic/ZvwIr8fJsnA/maxresdefault.jpg)