How to kill a witch without touching it?
Dig a hole, any depth you want. Put at least two pistons on the side, with a block attached to each of them ; build a mecanism to activate them both.
Lure the witch to make her fall in the hole ; crush her.
This can even be done very fast if needed : when in a pinch, first make here fall in a 1x1 wall, cover it so she doesn't throw potions, and you just have to dig again and put your pistons to crush her.
Or just use gravel/sand.
Use the Witches' tactics against them: make instant damage potions and throw 'em!