Here is the witch farm we build on our survival server.

- Clearing entire 256x256 area of spawn places (1/2 slabs and lighting).
- Witches spawn on shifting floors and they fall through.
- The floor shifts on a redstone timer, that sends pulses to the pistons. (any design will do).
- Then witches drop minimum 30 blocks to there death onto hoppers.
- Hoppers place the items into chests.