Hi! Go the right of Cerulean City and cut the bush. Fight the numerous trainers until you reach the end. You will see a patch of grass there. Don't worry about that. The only new pokemon there is Voltorb. Don't catch it, it is onlyaround level 17. You can catch level 40 in the power plant later.
Go down to the Pokecenter. Before going into the Rock Tunnel, I advise you to bring your Flash pokemon, grass or water pokemon, and psychic pokemon, Hypno if possible. Also, carry at least 5 pokeballs. Go in and use Flash. The pokemon inside are Geodude, Machop, Zubat, and Onix. Catch the first three pokemon at level 17 or 18. Onix is very rare. If you're lucky, you will fight it at level 17. If you fight level 15, catch it anyway in case you never fight it again. To catch it, use Hypnosis to put it to sleep. Then, use a grass pokemon that is not too high level and use Vine Whip. The best strategy I think is to poison it and attack it with Leech Seed. Leech Seed is a grass attack. It drains a little of your enemy's HP every turn to the current battling pokemon. But you can't make it sleep if you poison it. Throw a pokeball or great ball when Onix's HP is running low. Hurry or the Leech Seed and poision will kill it.
A lot of the trainers in the Rock Tunnel are Hikers. These Hikers usually start with Machop then end with rock pokemon so I advise you to put a psychic pokemon in first. Sometimes Hikers don't even use Machop, though. Rock Tunnel should be easy except it is pretty long. I advise you to bring plenty of antidotes and potions because you don't want to go back to the Pokecenter and come back again! When you finally beat the Rock Tunnel, there are four more trainers near the exit outside. Try to avoid some while you heal your pokemon in Lavendor Town.
