The way Atma work for me:
- join region you need an Atma from and do Fates for 20 min
- if there is no Atma then switch to another job and try another 20 min
- if there is still no Atma go do some dungeons

Got all Atma in couple of weeks doing fates about 30-40 min a day. Noticed that Atma drops for me when I actually do my job well (like having hate of all mobs whole duration of fate as tank or killing mobs super fast as dps). Also noticed from my friends and fc members that they got Atma much faster using jobs they are best at - so if you have multiple jobs with Zenith weapon, try to switch to other.

If you have just 1 job and people in dungeons still often try to give you advice on how to get better (or simply say that you suck) then your Atma grind might tike "a while longer".