I'm working with some friends, running tests on Atma's to find a pattern. Well I'm on 9 Atma's now, after around 4 hours of trying, which seems pretty quick compared to most people.

This is what I found, it's not quite a guide, but it's things we've tested and found to be pretty consistent;
  • The Atma's are (obviously) signs of the Zodiac, but the game doesn't put them in order. List them in order of the Zodiac, and start with the first one down your list that you don't already have.
  • They seem to drop in the first 10 fates,or after 40-50 - anything inbetween seems a waste of time. I've been doing 10 fates, if it doesn't drop, go do a dungeon then come back and do another 10 - rinse and repeat.
  • As soon as one drops, go to the next one in the list and don't do anything in between - just get straight to the next area and start doing fates. I did 3 Atma's in the space of 5 fates using this method, as did a friend, so it seems reliable.