Clay's Pokemon are at 31/31/33 (Krok/ Sandslash/ Excadrill). You need to level up more in order to deal with Excadrill.
Additionally, do you only have 3 Pokemon? You will need a team of six, regardless of you planning to get Kyurem, since you don't get it until after the Elite 4.
I had little problem with Clay's gym due to my starter (Oshawott), so what you need is either a Water type or a Grass type. For the sake of your game, get a Panpour since it will be able to use Surf and Waterfall, two HM moves that are essential to getting around and will be good STAB moves in battle. It will be in rustling grass at level 22 in Lostlorn Forest; Pansage will be there too.
While Marill would be better (with Huge Power), you don't get it until after the Quake Badge, on Route 6. Though you can get Azurill at Floccesy Ranch or surfing on Route 20, it's such a pain to level up because it's so weak. Even with Huge Power, it doesn't learn any physical moves until level 20.