Hmm,i have been thinking into incorporating a couple of achilles heels' to Witches.

More specifically, the Oddyssey's.

So there exists an herb bulb called Molly that when ingested or its smoke inhaled, it robs a witch of magic temporally. They might even show symptoms of poisoning and be crippled or die if it is an enough dose. If a normal human consumes the herb (besides getting an onionlike taste), they gain temporal immunity.

Also an object not of this world like, say, certain character's meteor sword is flat out impervious to magic.(This one isnt anywhere but could work)