I've been a Trekkie for nearly 20 years now and to be honest, from watching every single minute of the franchise, my grandmother could beat up the Klingons.
I mean they stumble around, stumping their feet, yelling, belching, screaming, swinging some ancient blade around making their presence known so that anyone with a Phaser could take them all out from a mile away from a nice elevated position.
In an episode of DS9 several of their best warriors are beaten down by a heavily pregnant woman whose been badly treated for days. A 8-9 month pregnant woman, short, thin, tired. Kicks the crap out of half a dozen Klingons, simultaneously.
Buffy would be overkill, her and her Scythe could take them all out, nevermind a handful of them.
Buffy the Klingon Slayer, good gig for her once all the Vampires are gone.