If you are looking for a world for this specific achievement, and since it's a bit tricky to import worlds on this version of the game, you can use the solution suggested by u/WaveSmashReddit
on Reddit (all credit goes to him/her):https://www.reddit.com/r/MCPE/comments/8t3rm6/easy_zombie_do...
I copy/paste the instructions just in case the Reddit thread happens to be removed:
"Make a new world using the seed "SNEEZE" of course without the "". Turn right and head straight into the snow. You'll find an igloo with a hidden bunker under the carpet, coordinates 820, 71, 29. There will be a Villager and a Zombie Villager in separate cages. You can take the golden apple from the loot chest and the splash potion of weakness from the brewing stand. Just throw the potion onto the Zombie Villager and use the Golden Apple on him afterwards. (You can't throw the potion at him through the iron bars. Break one of them)."
This is the seed I used to unlock the achievement, on the 1.11.1 version (05/22/2019). Please let me know if it no longer works with future updates.