Whilst Joker could have killed Batman many times over, the reverse is true. If Batman killed willingly, Joker would have been six feet under a long time ago. He's far from unbeatable. Not only that, but Joker not killing Batman is a huge weakness; he likes to toy and play around with his opponents far too much.
Anyway, let's flip this around and look at what Joker; a deranged, psychotic and incredibly dangerous man, is facing:
He is facing a Slayer.
By definition, a Slayer is a chosen woman with super strength, speed, reflexes, agility, enhanced senses, enhanced healing abilities and natural proficiency in combat, who (in Buffy's case) has been fighting Vampires, many different types of Demons and other supernatural beings since she was fifteen years old.
She's been thrown off of buildings, through windows, she's fallen five stories onto her face, been stabbed, shot, even had her blood drained by a vampire, and none of that's kept her down.
She's averted the opening the hell numerous times, she's kicked her boyfriend there (moments after she had witnessed him become good again), and she's even gone there herself and rescued a group of humans and returned, unaided. She knows how to use multiple weapons, and can stab you in the heart on instinct alone. She is the being that all of the stuff that mortals are afraid of fear.
Now, let that sink in. I'm not suggesting for a second that this is a cakewalk for her. Joker is bad, he's dangerous in many ways. But Joker has been shown to be susceptible to losing his cool. Coco (as much as it pans me to admit it) brings a good point up with Terry McGuinness; he talks trash to Joker and it throws him off, leading to his defeat. Buffy loves talking trash. If she talks enough to piss Joker off, and he gets in close and gloats as he's about to kill her (as with the Terry McGuinness example) then stake to the heart while he's monologuing.
It isn't impossible for Joker to win this, far from in fact, but I really can't see it.