Disclaimer: Harry Potter owned by JK Rowling; BtVS oughta be owned by Joss Whedon.
They stand alone in an English field before a lake and a castle. The children watch from windows and battlements, obscured to pinpricks by the distance.
"I can assure you that I understand the depth of your loss Miss, but I'm afraid I must demand you stop." He looked absolutely ridiculous, with a long white beard and his gaudy starry robe and the twinkling eyes.
"Demand? You demand I stop? You aren't exactly in a position to be demanding anything, old man." She's got the other classic look in the profession, jet black eyes and hair on pale skin with an option on veiny.
"I cannot allow you to -" She flicks her hand and the grass at his feet transfigures into thorny vines, reaching for his wand, his legs, his neck.
A mumbled word in Latin and suddenly the vines are gaudy green streamers, lifeless on the breeze. "Stupefy!" A flick of the wrist, and bright red light shoots out to meet her.
She's already cut her right palm open with a fingernail, and catches the stunner coming at her barehanded. Red blood mixes with red light and the curse turns black. It reminds her of her hair. She whips it back at him, even as he's shouting "Reducto!" and whipping his wand back around, "Vinculo!".
He watches her stepping to the side, grazed by the first bludgeoning curse and spun around. The writhing black light is coming at him slowly and he sidesteps, letting it plow by. The steel chains from his second spell are about to catch her, but she shouts "Thicken!" and they hit hard air and drop. With a complex wand gesture and a three part incantation, suddenly a rock just behind her is an angry lion, and it's pouncing for her, still off balance and stumbling from his bludgeoner.
She smiles as the cloaked black homing spell, made from the fool's own magic, smacks into his back after pulling a 180 just behind him. His scream of pain doesn't seem to bother the lion though, and as it pounces she fights it hand-to-hand, her fists glowing with blue fire and torching its dirty mane.
His vision darkens and he feels so tired, but he manages to stay on his feet and throws out a desperate flurry of small spells to distract her from the lion: a cutter, a blinder, another stunner. Even as he casts the last he sees the lion dead, feels his energy draining deeper into the black poison she turned his first stunning curse into. Desperately, he starts a general counter curse for fatiguing magics.
She's nicked by the cutter (more blood, still red), but it doesn't matter, the lion is dead and the wizard is on his last legs. She reaches inward and feels her pain and black chain lighting races out of her fingertips to kill him.
He isn't there anymore. A clap of air and he's at her side, free of her spell and twirling his wand. She's too slow to react and "Argens!" conjures a quicksilver rope which catches her around the arms and knocks her down with the force of the blow.
Bleeding from a half dozen minor wounds and at his feet, she reaches out for branches, rocks, even a field mouse with her telekinesis. His off hand moves back and forth, banishing each projectile as fast as she finds a new one.
He stuns her and her eyes start to shut but it looks like she's going to shrug it off, even with the silver anti-magic bind still wrapped around her arms. He takes a step closer and starts a longer, more complicated spell to knock her out completely, and then he notices the silver rope is cut.
She flips to her feet with a move she's seen Buffy do a thousand times, and drives her right hand into his chest.
Everything feels so distant suddenly. His wand feels wrong as it drops out of his hand.
She catches it left handed and spins it like a baton. "Tastes like lemon drops."