If Atma doesn't drop within the half an hour you have 2 options:
1) switch to another job that has Zenith weapon - try another half an hour
2) go to another region - try another half an hour
If none of 2 work stop wasting your time and do dungeons, crafting, gathering or something else. Try again after couple of hours again and should be instant drop.
This did work for me and I spent very little time and had fun while getting Atma.

Just don't waste time trying to get certain Atma for more than an hour total.