Fighting with Farnese

To get Farnese in my party, do I need to just duel her, or demand battle and fight with her party? If so, I messed up, cuz I'm a vassal of Holy See, and I don't feel like just being released from oath and get relation with 'em back to 0, so I can battle with her.

You have to defeat her party, which would probably involve not being in the holy see.....

Oh great...