Incidentally, the Frost Cavern is actually a really good place to train Pokémon for the Anistar City gym, whether you're battling trainers or wild Pokémon.
If you'd prefer a place to train with lower level wild Pokémon, Route 16 is relatively close by. Just for reference, the Anistar City Gym is a psychic type Gym, so training Pokemon with Bug, Ghost, and Dark type moves is a really good idea.
If you'd like the option of running away to a Pokémon Center when battles get dicey, I'd recommend fighting wild Pokémon. It also might be useful to have Pokémon with Dig and Fly handy for a quick escape from the cavern, or just a mon with Fly and a collection of escape ropes. I'm still willing to trade some Pokémon to you if you want, to help your team. I also have Pokémon with Dig and Fly, if you just want those.