To all those farming Atma's - from what I did and saw (have Atma weapon now) and heard from other people as well.

You can get Atma with bronze, silver, gold. It doesn't matter which one you get, atma will drop.

For FATE grinding part of atma, just kill few mobs, boss/NM fates, just hit the boss for maybe 5%-10% ish and just leave the fate to another fate.
It's better for you since you can grind other fates that may be up in the area at the same time gives low level players that are leveling through FATE's a chance to get silver/gold. It's more work for them to kill mobs or hit the bosses with higher HP but this keeps both sides happy imo.

I just leave right after few kills instead of staying for the full fate and still got atma drops.