After you venture on all of the Sevii Islands, bringing the Ruby and Sapphire to Celio at the Pokémon Net Center on One Island, you have access to enter the Cerulean Cave. The Cerulean Cave is home to powerful Pokémon with mazes of water. Go to the B1F of the cave and you’ll find Mewtwo at the left end. Save your game before you interact. Mewtwo is on Level 70. It will use moves like Swift for a never-miss strike, Psychic to lower the target’s Special Defense stat, and Recover to restore HP. Use moves that cause status problems, like Thunder Wave or Sleep Powder and Ultra Balls for better capture results. ... https://www.youtube.com/watch?v=o-drbjlRRLU
After defeating the Elite Four, you’ll take on Lance, the league champion. Gyarados will use Waterfall for a 20% chance of making the target flinch and Flail to deal more damage if its HP is lower. Dragonite will use Thunder Wave to paralyze the target and Dragon Rush for a 20% chance of making the target flinch. One Dragonite knows Thunder that has a 30% chance of paralyzing and the other one knows Blizzard that has a 10% chance of freezing the target. Aerodactyl will use Rock Slide for a 30% chance of making the target flinch and Aerial Ace to strike without fail. Charizard will use Shadow Claw that has a high critical hit ratio and Air Slash for a 30% chance of making the target flinch. The 3rd Dragonite will use Safeguard to prevent status problems for 5 turns and Fire Blast for a 10% chance of burning the target. After the battle, you’ll enter the hall of fame and unlock new quests for later.
...
https://www.youtube.com/watch?v=3_EY00kSy-8
At the safehouse in Driftveil City, Hugh will be there, and you can battle him there once per day. This round, his team is at higher levels and their movesets have updated as well. Unfezant will use Giga Impact that requires recharging the next turn. Flygon will use Draco Meteor while the Dragon Gem powers it up for once but its Special Attack stat will be sharply lowered when using it.
...
https://www.youtube.com/watch?v=pUqAeMe-IaM
As you explore Victory Road, you’ll battle many trainers and use Strength to move boulders to the panels to unlock certain paths. As you explore, you’ll come across another legendary bird Pokémon, Moltres. Save your game before you interact with it. Moltres is on Level 50. Its Fire Spin can immobilize you as it repeats itself for 2-5 turns. Using sleeping-inducing moves and Ultra Balls would be recommended.
...
https://www.youtube.com/watch?v=5xI33WkDYSE
After taking care of Team Plasma in Castelia City, go to the gym where you’ll see slimy walls you can slip through and step on switches to form a path. After forming the path, you’ll battle the gym leader. Burgh uses Bug type Pokémon. Whirlipede will use Pursuit if opponents try to switch out and Poison Tail for added chances of critical hits. Dwebble will use Smack Down to knock down those in the air and Faint Attack for never miss strikes. Leavanny will use Razor Leaf for critical hits along with Protect to avoid attacks for one turn. After the battle, you’ll receive the Insect Badge and a TM76 (Struggle Bug). Struggle Bug strikes all nearby opponents and lowers their Special Attack stat.
...
https://www.youtube.com/watch?v=TyvmvaKpf0M
This level takes place underwater. It’s the same as World 2-2, but this time, there are more Bloopers and fish than before. Collect the coins as you swim by and use the pipe at the dead end to return to the surface so you can use the big stairway to reach the flagpole.
...
https://www.youtube.com/watch?v=2YXl2p2lPeI
In this underground level, cross a big hole and you’ll encounter 3 biscuits in a garbage block tunnel. Hit invisible blocks and the garbage block above them to reveal a vine that will take you to a Warp Zone so you can go to either World 6, 7, or 8. If you’re still underground, you can go down one pipe for coins or continue ahead by crossing platforms going down. Continue ahead and you’ll see a Buzzy Beetle, which cannot be harmed by fireballs. After 2 ladybugs under a set of garbage blocks, you can stay above ceiling to go the Warp Zone that leads to World 5 or use the pipe by the wall to return to the surface so you can use the big stairway that leads to the flagpole.
...
https://www.youtube.com/watch?v=jxQ2g3oPDA4
After meeting the legendary Pokémon at the Burned Tower, they will roam all over the Johto region. If you encounter one of them, a page to your Pokédex will be added. Suicune is on Level 40. A roaming legendary will flee on the first turn. Any Pokémon like Gengar for example that knows Mean Look can prevent wild Pokémon from escaping battle. Even if the roaming opponent can’t flee, be careful as it can use Roar to force your Pokémon to flee. Try using a combo with Mean Look and Hypnosis then use Ultra Balls. If you have a Master Ball, you can snag the roaming legendary with ease.
...
https://www.youtube.com/watch?v=QZlD3IVw97Q
Cerulean City’s gym leader, Misty is an expert on Water type Pokémon. Starmie will be the hard part as it has great speed and Misty will sometimes use X Defend items on it. After winning the battle, you’ll receive the Cascade Badge and a TM11 (Bubblebeam). Bubblebeam can sometimes lower the target’s Speed stat.
...
https://www.youtube.com/watch?v=x2qPSni1Kbk
After you go through numerous panels in the Rocket Warehouse on Five Island, you’ll reach the tech area where 2 admins are monitoring. As you enter the tech area, you’ll battle the female admin, whom is mentioned to be Ariana. Muk will slow opponents down with Rock Tomb, Minimize to become harder to hit, and Sludge Bomb to poison targets. Arbok will use Iron Tail to lower the Defense stat of targets and make them flinch by using Bite. Vileplume has 2 powder moves that can paralyze or put targets to sleep along with Giga Drain to drain HP from foes. After the battle, the admin will reverse the arrow tiles.
...
https://www.youtube.com/watch?v=fYt7ME4ofr0