This is best to combine with the achievements Losing Your Religion
using the Map editor.
Refer to this guide for all 3:
If you just want this achievement quickly:
Go to Single Player
-> Custom Game
Change Game Type
Change Starting Age
to Post-Iron Age
Change Map Type
to Hill Country
Change your Civilization
Ensure the tick boxes for Full Tech Tree
& Allow Cheats
1) Enter the cheat code Steroids
2) Get a villager to build a Temple
3) Get another villager to build a few houses (Hot Key Q
for faster selection)
4) Recruit several Priests
from the temple
5) Enter the cheat code Hoyohoyo
(super speed priests with a lot of health)
6) send your priests out to convert units - only use 1 priest per unit for conversion to save time.
7) after 20 units have been converted the achievement will pop.