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.

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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.

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Or just use gravel/sand.

Use the Witches' tactics against them: make instant damage potions and throw 'em!

Just Make A shield and whenever a witch throws a potion at you deflect it towards herself. that ways you dont waste any resources on her (almost).

Dig a 2 block hole, push the witch inside, and crush it with a piston.

-You can build a tower high enough for fall damage to kill a mob and then use a fishing rod to yank mobs up to you and watch them fall to their death.

-You can make a cheap slime block piston launcher to toss mobs into a pool of lava or a pit.

-You can do the classic piston mob crusher. (I´ve had problems where mobs would walk through blocks and escape before though)

-You can use a tnt duplicator glitch and blow up mobs using an obsidian floor.

Hope this helps add more weapons to your arsenal.