I haven't played Ranger in a long time, but I believe it's in the Go-Rock Squad's hideout, the one that was hidden under a rock and blewup. You know,the one with security sensors? A Metagross is in the last room.
Metagross is a pain. It takes about 30 loops, and before you can loop all of the required loops, It'll fire its painful Hyper Beam. BE careful.
BTW, welcome to Psypokes.