My conclusion on Gravity and RainDance in conjunction.

Well, thing is, would you expect Thunder from Jirachi? Good chance it does if I'm switching it out @ Gyarados. However, then assuming they do switch to Hippowdon, so what? Ok, Sandstorm negates Rain Dance instantly. So Jirachi sets up Gravity. What can Hippowdon do? Spam STAB'd EQ? Unless it's an Adamant nature one with maxed att EVs, I'm not too worried. I'll spam accurate-Dynamic Punches at it to confuse it, thus forcing the player to switch to something else, or he/she will use Roar at me. Technically, it could go either way here.

A good player would figure out the gimmick with time. Gravity isn't completely unheard of, just more difficult to use. Hippowdon was the first thing that came to mind, a bulky Tyranitar could do much the same.