Good luck on this one, literally. The green bats are random, and VERY rare. There are people who have played hundreds of hours and not gotten this achievement.
Also, it appears that you CANNOT earn this in Seeded mode, even with three different seeds (killing three "different" green bats).
This is more of a warning than a guide.
The one tip I can give is that occasionally green bats will show up in the daily challenge, which means you may be able to keep an eye on reddit/twitter/wherever, and fire up the game if that happens. Then do it again two more times.
