While you'll freely admit that you may not be the most well educated in the fine differences between the various fields of magical study (you wouldn't know the difference between an enchanter and an artificer), if you've learned anything from Zercoa it's that potioneering is a subskill of witchcraft (voodoo witchcraft in the zebra's case). And if there's anything you know from fairy tales, it's that witches have two very big weaknesses.
One is being shoved and then locked inside flaming ovens. And while the presence of six massive ovens in the same room might be something of a clue, you're honestly not too comfortable with the idea of throwing a pony into a fire (and then watching them scream and burn like it's the Saddle Witch Trials).
Which is why your plan is option number two.
Before she can to anything terrible to you, you whip out your bucket of well water and fling the contents across her.
"Ha! Why don't you try this potion then!"
The witch looks back at you, very wet and with pond weed draped over her head, but otherwise no worse for wear. She licks her lips. "Mm. Interesting flavor."
It didn't work. Why didn't it work? As you start to panic, she continues her analysis of your failed attack. "I'm getting notes of algae, rot essence, is that a trace of ink I detect?" The short witch sucks on a lock of her mane. "Definitely ink. Strong undertones of fear with a dark magic base. Seems stagnant though." She smacks her lips. "Left it in the alembic too long? Naughty, naughty. No wonder it's gone off."
She looks up at you and fires off another face-splitting smile. "Not bad work, overall! If you trained under me for a few years, I might be able to make a decent potioneer out of you. Buuuut then I wouldn't get to harvest some rare pegasus ingredients. Oh well! I didn't need an apprentice anyway. Now come'ere!"
She lunges at you, only to immediately trip and fall on her face. You lean down to her. "Uh... you okay?"
"Never better!" she replies cheerily as she unsticks most of her face from the floor. "Lemme just check something." She lifts her robe then drops it again. "Ah, thought so. It seems I no longer have any legs. Bother."
That's not all she doesn't have. You've been watching her slowly melt like a candle in the sun even as she acted completely unaware of it. It would have been much grosser if you could see all the bones and blood, but she seems to be the same color all the way through.
"Oh rats," she muses as her face starts to merge with the puddle that's become of the rest of her. "I'm going to leave a terrible stain."
With those final words, the last of her melts away into a pool that's quickly draining away through a crack in the floor.
A bell chimes somewhere above you, signaling your victory. You eye the puddle one last time. It seemed like a cruel way to go, but if she never even noticed, at least it was painless?
Before you can dwell on that too long, a glowing light illuminates a ratty broom resting beside her cauldron.
"I guess that's my prize?" When nothing happens to contradict your theory, you go and take it, stuffing the Broom away into your helpfully expansive Inventory Bucket.
"What a weird pony," you remark as you hop over her remains on your way to the door. "Feels like they took the worst parts of Twilight and Pinkie and fused them together. All the crazy, none of the caring. All the magical study, none of the logic."
You can't wait to find out how the two of them'll react when they encounter her.