The Cerulean Cave is a dungeon located across the river to the northwest of Cerulean City. It is filled with very tough wild Pokemon, so you will not be able to enter until after you become Pokemon League Champion. Return and speak with the man guarding the cave and defeat him in a battle to gain entrance.
There are a lot of high-leveled Pokemon in the Cerulean Cave. It’s a great place to chain Chansey or other Pokemon to quickly level up your team. Also, check every spotlight through Cerulean Cave for hidden items that respawn every hour. Keep an eye on your Partner Pokemon’s tail.
A river will twist and turn from the starting point at the southeast corner of the first floor. Follow the river and get off at each small island to find an item. The first item is a PP Max at the northeast corner of the floor.
Surf along the path to the northwest until you reach a Max Repel near a ladder.
Walk south towards the river to find a Full Heal. Finally, follow the path on land east and north. This will lead you towards a Max Revive. Those are all the items on the first floor. Take any ladder up to the second floor.
The second floor of Cerulean Cave is completely open, unlike the original layout. Interact with the different spots of light to find various hidden items that regenerate every hour.
The best hidden item regenerates once a day and can be found by interacting with the large crystal in the middle of the floor. This will give you either a Max Revive, Helix Fossil, Dome Fossil, or Old Amber. The last three items will revive into an ancient Pokemon.
You can also grab a handful of regular items including a Max Lure to the north, a Rare Candy near the southwest corner, 5 Ultra Balls near the south side of the floor, a Max Revive near the southeast corner, and a Full Restore near the previous item.
Walk to the northwest corner on the second floor and take the ladder down to the first floor. This should bring you to an enclosed area with one other ladder. Take this second ladder down to the basement.
The basement of Cerulean Cave is a winding path similar to the first floor, but you will start off on the northwest corner. Follow the rocks to the northeast corner where you can grab an Escape Rope.
Continue along the path to the south, and circle up the hill to the north. A Full Restore can be grabbed near the edge.
The path will circle around towards the middle of the cave where a Max Elixir rests.
A small lake can be found to the west, passed the hill. Follow this lake to the west where the strongest wild Pokemon in the game will be waiting. You can also find a PP Max in the alcove to the left of the wild Pokemon while surfing.
Mewtwo is a high-leveled psychic-type Pokemon. He can easily crush your team if you’re not prepared. Do not use any Pokemon who are weak to the psychic-type or who have low special defenses. That includes fighting-type and poison-type Pokemon. Remember, psychic-type Pokemon are weak to ghost-, dark-, and bug-type attacks; and the reward for defeating the Coach Trainer was the strongest bug-type attack in the game, Megahorn!
You will have five minutes to defeat Mewtwo. If you run out of time or your entire team falls to this beast, reload your save file and try again. If you forgot to save your game, you’ll have to beat the updated Pokemon League to find Mewtwo here again.
Mewtwo’s amnesia will boost his special defense even higher, so it’s best to stick to physical attacks and Pokemon with high attack stats. Make sure one attack does enough damage, or his recover ability will restore half his health each time it’s used. Furthermore, this Pokemon’s swift attack will always hit your Pokemon, so it’s pointless to increase your evasiveness or decrease his accuracy.
After the battle, you will be able to catch Mewtwo. It’s best to use the Master Ball from Silph Co to catch him with a single throw. If you no longer have the Master Ball, use Golden berries and Ultra Balls when the ring is at its smallest. Do not try to throw any balls while the purple barrier is up.
At the end of the match, exit out of the cave to meet up with your Rival. He will give you 3 Escape Ropes and tell you about some mysterious girl who was looking for Mewtwo. Return to the spot where you found Mewtwo to meet a girl named Green (originally named Blue in Japan). She will immediately challenge you to a Pokemon battle.