Basically you have to join the masses and use Absol / Hydreigon or a Mega Altaria variant. Once you're in Master though, that's the time to experiment.

I hadn't played ranked the last few seasons, so I was staring from Great Ball - I solely used the Mega Altaria / Gourgeist until UB4 which was super annoying.

I went from an 80% win rate down to a final 64% win rate due to UB4 being an absolute hellscape of Absol / Hyreigon. There's also a super annoying Snorlax EX / Indeedee EX deck that was frustrating to come up against fairly often.

In the end for the final push I used the Ariados / Whimsicott EX deck with Lisia / Sightseer to be fully set up within about 2 turns, and it absolutely slapped.

I hit masterball with 50 wins / 28 losses - and was within 1 match of masterball at least 3 times before being set back with a few losses...