This is a low-budget team which isn't affected by RNG factors.
"The Unleashed" Polaris Mask is the core that makes this challenge possible. He is adaptable to "SPD Down" debuffs due to the additional action granted by his 2nd skill. His 3rd skill is especially useful in this fight: cleanse the "Invincible" buff in the 4th turn to help Kyoichi deal additional damage, apply 2 stacks of "ATK Down" debuffs while Phein is generating VP, and comsume redundant VP when needed. Also, he should have higher stats because you borrow him from your friends. That makes survivng the first turn without "ATK Down" possible.
Phein applies 2 stacks of "ATK Down" with his 2nd skill, which makes him a solid pick. He also serves as a VP generator because he's the only unit who gets spare time. Unfortunately, you can't rely on his debuff once you decrease the boss' HP to less than 50%. From that moment on, the boss' auto attack removes its own debuff per hit. This is the most deadly mechanic in this fight.
I chose Kyoichi as my attacker. There's literally no alternatives. Note that Kyoichi would have been able to cut down half of the boss' HP in the 6th turn if he had boost. In that case, Polaris might not survive in the 7th turn depending on his setup. It should still be fine because Kyoichi would kill the boss in the 10th turn.