Three more tech alternatives , assuming your base is Gengar / Nidoqueen and a minimum of 2 Moonlights to bring or retreat Queens at will :
- Ditto + a couple of multi energies : you can dragon pump them back and get cured between turns ...
- Potergeist Rotom ( works with psychic energies ) + Lucario GL in bench ... with one or two prior shadow rooms , one of these with a couple of trainers in hand take them out
- 2-2 Manectric PL + Lucario GL ... if benched, no dragon pump splash on your bench .. if active , spreads to G'd Claydols , Uxies, and Cycler Bronzongs the same , crippling their resources , or 80 with two energies and Lucario GL benched ( and gets cured in the process , with free retreat also )
You can build an optimized Gengar Queen for Kingdra , but that may weaken you for other matchups ... that's the beauty of this game. And for Nats there will be no metagame as you will face people from all over .. but ditto or Poltergeist Rotom + Lucario GL are not expensive alternatives.
One of my kids constantly faces Kindgra in top cuts and I've built for him a solid Gengar-Queen list . it's not easy but he wins most of the time. If you run a SP based deck ( like my other kid does ) , a Luxray GL based deck with Lucario GL benched just destroys kingdra .
Good luck.