Sierpes really does have a gigantic HP bar, or did I get here too early?

I came to defeat her right after solving the problems with Diosdado's noise and the secret door (yes, I kind of used a guide to get the true ending... sorry), but Mea Culpa seems like a toothpick against her. Either her health bar is gigantic or my damage is minuscule. If it's the second option, when is it advisable that I come back here? I've already unlocked Fast Travel through Prie Dieu, so I wouldn't suffer having to do the whole path again.