Requirements for this achievement:
- Splash potion of weakness
- Zombie villager (so not a regular zombie) --> difficulty has to be easy or higher
- Golden apple
The hardest thing for me to figure out was the controls. Apparently you need to have the splash potion of weakness in your hand, then aim at the zombie villager tap and hold your finger on the zombie villager. The potion should be thrown and grey sparks will emit from the zombie villager. Then equip the apple and tap and hold again (seems like you are eating it).
The grey sparks will turn grey and the achievement will unlock.
I've posted a world with a setup for this achievement. I did not create this world and do not take credit for this world. I just trapped the zombie villager and made sure he could not die due to the daylight. Achievement worlds