This all depends on the Grail system used in the ensuing battle though...
How so? Illya can support Berserker even without the Grail (she canonically summoned him a few months before the Grail War even started).
Plus, it's outright stated in Berserker's FGO interludes that FGO-Berserker doesn't have God Hand and has had his Mad Enhancement reduced (which is why he can use Nine Lives) to conserve power because Chaldea can't support him at full strength. Regardless of whichever Grail System is used in the following battle, Illya can give Herk his full power, and FemMC can't.
The way I see it, at best FemMC's Berserker takes nine of Illya-Berserker's lives, then God Hand resurrects him now immune to Nine Lives and FemMC is screwed. Maybe if she uses up all her Command Spells she can boost her Herk's Nine Lives up a Rank so that it'll kill Illya's Berserker again?
Jokes on her, as long as FemMC have saints quartz she can revive her herc as much as she wants and every revival comes with a fully charge NP. It's cheap and it's cheating, but that's just how it is.
