In a short Q&A with USA Today, Joss Whedon was asked who he thought would win in a fight between Buffy the Vampire Slayer and Natasha Romanov, the Avengers‘ Black Widow. His response is a very entertaining action sequence all by itself:

Buffy would go easy at first, but as soon as Natasha popped her with a Widow Sting, she’d start bringing some slayer brawn to the fray. Natasha’s fast, but a couple of good connects and she’s wobbling, possibly something broken — she whips out her glock and now Buffy’s dodging — right where Natasha wants her. Natasha shoots the cable holding the steel barrels and they tumble onto Buffy, nearly burying her — Buffy just arcs out of the way, grabbing the splintered cable and swinging directly onto Natasha, a bullet grazing her cheek as her feet land hard on the Russian’s shoulders, sending her back flat — crack! — on the floor, Buffy wrenching the gun away and tossing it, fist ready for the final strike. Natasha, struggling to stay conscious, says the fight’s over. Buffy agrees, but Natasha explains: She poisoned Buffy hours ago. That waitress that brought her salad …? Natasha smiles. The poison is dormant — ’til it’s activated by adrenaline. Buffy’s eyes narrow. “Too bad I didn’t use any.” Wham! Natasha’s out for the count, and Buffy’s heading — slowly — to Willow for a mystical cleanse.

That’d be my first guess.