QWhen my Oshi holomem is hBP04-006〈Oozora Subaru〉and my center holomem as well as 1 of my back holomem are hBP04-067〈Oozora Subaru〉, my opponent used hBP01-087〈Kobo Kanaeru〉's Arts "Rain Mantra".
If I use the Oshi skill "Mental! Physical! Passion!" when my back holomem hBP04-067〈Oozora Subaru〉is dealt 20 damage by the ability, can I -30 damage from the 40 damage dealt to my center holomem hBP04-067〈Oozora Subaru〉after that?

ANo, it cannot.
As the ability can only reduce 1 instance of damage, it can only reduce the damage at the timing that the Oshi skill is used.