Disclaimer: Buffy, Willow and Co. all belong to Joss Whedon and ME
Rating: PG-13 for language. If you can watch the show, this shouldn't be a problem.
Timeline: Set during Two To Go in Season 6
Author's Notes: Quite frankly, I was embarrassed for Buffy when she declared she was going to show Willow what a Slayer really was and then fought in a normal fashion before getting her ass kicked, so I decided to remedy it.
Some details may be inaccurate, as I am going by memory.
Buffy faced Willow grimly. "A killer isn't a Slayer. Being a Slayer means something you can't conceive of."
Willow shook her head, tilting it playfully so that strands of unnatural black hair fanned across her shoulders. "Oh, Buffy. You really need to have every square inch of your ass kicked."
"Then show me what you got. And I'll show you what a Slayer really is."
Willow laughed derisively. "You're so full of crap, Buff. You should know by now that all a Slayer really is a tool. Not a mechanical tool, but still a tool, and a tool can only do so much. Power? You don't have power beyond your strength. I can sense power. I am power."
Ceasing the languid circling they had been doing, Buffy set her feet, and smiled grimly. "And that's where you're wrong. You see, I am the Slayer. I tried so hard to escape that. But it's who I am. It's ingrained in my body, my mind, my soul. The power I draw on can never run out or be displaced. But yours?"
She waved a dismissive hand. "You're like a kid that's found a machine gun and decides to get rid of that nasty girl who says her pigtails are longer than yours. You're on a power trip- you're enjoying yourself. Me? I'm afraid of mine. But desperate times call for desperate measures."
Willow's eyes had flared with a scarlet glow at the point when Buffy had flouted the previously redhead's powers, and she snarled, "Let's see them then."
Buffy titled her head back, and focused her mind of diving into the depths of her being, into the source of darkness there, and drew upon it, in a long, slow sip. Willow, watching her, saw with shock, worry, and no little disbelieving trepidation as the Slayer's head jerked downwards sharply, and then rose slowly, her eyes flashing a distinctive, vampiric golden, before returning to their usual glinting emerald.