Having done some genderless mons recently, the strategy varies based on how expensive it is to find the pokemon and how easy they are to catch.
Here are a couple examples:
Magnezone
Magnemites are easy to catch, so I just went and hunted a bunch of them, then caught a bunch of dittos (and bought a few dittos). 1x31 dittos are cheaper than 1x31 magnemites generally.
Metagross
I initially tried to catch beldums, but they are a PITA to catch. (Need a ghost false swiper because they have takedown). It took a little while but I caught 8 of them. I kept even the ones with bad IVs.
Then I caught a bunch of dittos until I had enough 1x31s. From there you do a bunch of breeds between ditto and beldum to get 1x31 beldums. These cost 10k each (for the braces).
From there you breed half of the beldums with 1x31 dittos to make sure you have 8 1x31 beldums. Then you can breed between beldums and dittos the rest of the way. In my case I was only doing a 4x31.
Rotom
I did the rotom on a bit more of a budget, which is why you see a lot of 30s. I caught a few rotoms that had 2x30, so I figured it would be a good starting point. This one I bought up a few 2x30 dittos and and rotoms off GTL.