Evoland Legendary Edition

I think there is a soft lock on this achievement. If you try to do this after , it doesn't appear to spawn A'tuins anymore in this location or anywhere else I can see. Instead you get Wasps that spawn in it's place. So unless there's a specific area that spawns them, you'd need to start a new game, make it up to this area, then grind them out to get the achievement.

They will still spawn after but the spawn rate is significantly lower. They seem to spawn once every twenty battles in the forest sections west of the village. It's highly recommend to go for the achievement right after Kerith gets the "X-Crystal" attack.