Hello,
@Cyracius!
You usually just need to push the zombies into a pool of water, wait about 30 seconds, and they will turn into a stack of Drowned mobs. One stack only counts as one kill unfortunately, so you will technically need to kill 100 “stacks” of turned-zombies to complete that challenge. Zombies don’t always turn into drowned mobs; but if you are having issues with them never turning, I can certainly report this as it may be an issue.
I know
@spursszn had a good design for spawning the mobs in, pushing them through water into a deep pool, which allows them to spawn several stacks to speed up the challenge. He would likely be able to explain it better/give some insight on this!
Have a nice day/night!
(P.S; good-luck with that challenge)