I got a claw fossil and resurrected it, but Anorith levels up very slowly and it takes so much exp to level up so what is the best grinding spot for this Pokemon?
Anorith has erratic leveling rate, which is the slowest leveling rate until it catches up to slow at level 38.
If you care only about experience, then just give it an exp. share and have other Pokemon battle in-game trainers. If you care about EVs, then see Pokemaster's questions about EV training locations.