I really like Yanmega and I'm trying to build a deck that showcases its usefulness as well. To help deal with ability lock, I was looking at cards that can help you manage your hand size when you have to use Pokemon Ranger as your supporter instead of Judge. Shaymin, Octillery, maybe possibly Empoleon would be good partners. Worst part is you can't get around Hex Maniac no matter what. I'm worried though, it could get to a point where you have to take up deck space on cards that cover Yanmega's shortcomings.
Another issue is you have to consider when you should stop playing cards, as you have to guarantee having 4 in your hand to benefit from having Yanmega, which can feel restrictive.
On those turns that you can't get around ability lock, it would help to have something like GolduckB to shuffle energy around as needed. If AromatisseXY was still standard, you could make a useable Fairy Transfer toolbox deck as well.
But that's assuming you don't have another solid attacker that you can just accelerate some energy onto. The beauty of Yanmega is no retreat cost or specific energy reqs, so you could probably invest in something like ZygardeEX and Strong energy and retreat when you're ready. Something tells me you'll have a pretty easy time no matter what you pair with Yanmega, unless there's another deck that deals gobs of damage besides MegaRay.