Simple, smash your way in and fight.

"I say we just smash through the window and ask for the diamond. If she wants a fight, we'll give it to her."

"Sounds like a solid plan. Let's go." Batgirl throws a batarang at the window, smashing it before swinging down and through the window. You follow. "Give us the diamond and you won't get hurt!" She shouts to the surprised Catwoman.

Catwoman starts laughing, "Forget something, Batgirl?"

"I'll take that as a no!" Batgirl leaps towards her, throwing a punch. Catwoman's fast reflexes allow her to dodge. She gets a scratch in on Batgirl's exposed midriff. You decide it would be best if you helped out. You use your powers to slow Catwoman's reflexes. She doesn't manage to dodge the roundhouse kick that knocks her to the floor. Batgirl follows up by straddling her and punching her in the face a few times.

"Where's the diamond!?" She yells.

"It's in the bag over there, yeesh! Stop hitting me you crazy naked bitch!"

You check the bag and find the diamond. You then call the cops while Batgirl ties Catwoman to a chair. "What do we do now?" Batgirl asks.