The only "Theory" I ever subscribed to was that fate participation influenced if you got atma or not. I only think that's a thing because myself and another got 13(We both duplicated one) in the span of two weeks only farming for about one hour a day. When they didn't drop we moved to another area, or another piece of content. If one got one first, they swapped weapons and kept on until the other got it, or just "hung around" until the other succeeded. I would imagine since we got ours prior to the nerf that it's even easier than that now. So yeah... no special tricks or gimmicks... Just do the fates right and hope for good rng. You'll get it in no time if you're patient.