Designing an XP Farm in Minecraft BE

I'm trying to design a mob farm in Minecraft, one that is used for XP farming. Unfortunately, I don't know how to make it work efficiently. It needs to be able to constantly produce mobs and push them into a shaft that is deep enough to kill them when I'm not using it. I'd prefer it if I could do this without a mob spawner so I don't have to try and fight my way through mobs to get to the one I found or be limited to just one mob. A guardian farm would work best, but would be a pain to make. Finding them would be hard enough in Bedrock Survival.

I'd love to do a Witch farm, especially after Witches caused me so many problems in Survival. I once died and lost part of my armor and a pick to a witch, and then gotten poisoned by witches twice yesterday while lost in a cave and had to be bailed out both times. My rescuer even lost a lot of supplies after dying to the first one. The trouble with a witch farm is finding a hut.

I'm still trying to find desert so I can get four cactus blocks to make my original farm design, so if I get this farm working, I can stop looking because it will do both for me.