1. Obtain the National Dex: After beating the Elite Four and becoming the Champion, you need to travel to Littleroot Town and speak to Professor Birch. He will give you the National Dex, allowing you to catch more Pokemon.
2. Find the Slateport Gym Leader: The last gym in Pokemon Sapphire is located in Slateport City, but the Gym Leader, Juan, will not be there. You need to find him first.
3. Explore the Seafloor Cavern: Juan can be found in the Seafloor Cavern, located on Route 134 south of Slateport City. The entrance to the cavern is underwater, so you'll need to use Dive.
4. Defeat Team Aqua: Juan will be battling Team Aqua in the Seafloor Cavern. You must defeat them before you can challenge him to a Gym battle.
5. Return to Slateport Gym: After defeating Team Aqua and Juan in the Seafloor Cavern, you can finally return to the Slateport Gym and challenge Juan for the final badge.
Note: You can access the Seafloor Cavern before you defeat the Elite Four, but you will need to travel through the cave and defeat Team Aqua to access the Gym.
Tips:
* The Seafloor Cavern is a challenging area, so be sure to train your Pokemon beforehand.
* Juan specializes in Water-type Pokemon, so bring Pokemon that are effective against Water-type moves, such as Electric, Grass, or Rock types.
* Don't forget to use your HM moves like Dive and Strength to access areas and battle against the gym leaders.
Good luck defeating the last gym and becoming the ultimate Pokemon Trainer!