Then don't use it. Don't get roped into feeling you need a certain type to defeat Liza & Tate, especially a crappy Dark-type. If you truly want to take advantage of types, you can add a Shiftry or Cacturne to your team. As Grass/Dark, it will resist almost any attack Liza & Tate can throw at it. Personally it is stupid to me to have to raise a certain Poke just to take on a certain weak trainer. If you wisely picked Mudkip, Swampert handles the creepy twins easily. Any Flying/Levitate Poke will do well or any Poke that is fast and can dispose of Claydol before it can use Earthquake. Even a Poke that can take physical hits has no problems. A quick Poke with a status effect (Breloom with Spore or Gardevoir/Milotic with Hypnosis) can stop Claydol in its tracks too.
The main strategy in this 2v2 battle, is to focus both attacks on Claydol. Do that and you win.