Skyblock At what height does it take for a level 19 Zombies to be one-hit?

[totalledtitan95]

Just got back into playing minecraft recently and decided to continue playing my old save to see that i have an old level 19 zombie grinder that is no longer one shot-able, and deciding to re built it.
What height do i need for level 19 Zombies?

 

[Dualninja]

I don't know, but I would build about 25 blocks up and see how much damage it does. If it kills them instantly then build a platform for them to land on to reduce the height. If they don't die in 1 hit, dig into the Island until you can 1 hit them, that's what I did for my farm at least.