After defeating the Elite Four, you'll face the Hoenn Champion, Steven Stone, who specializes in Steel-type Pokémon. Steel-types are weak to Fire, Fighting, and Ground-type moves. His formidable team includes a Skarmory, Aggron, Claydol, Cradily, Metagross, and his ace, Mega Metagross. Skarmory is Steel/Flying, Aggron is Steel/Rock, Claydol is Ground/Psychic, Cradily is Rock/Grass, Metagross is Steel/Psychic. A strong Fire-type Pokémon like Blaziken or Camerupt will be invaluable. Ground-types like Swampert or Garchomp (if you have one) are also excellent, especially against Aggron and Metagross. Fighting-types like Hariyama or Machamp will also do well. Be prepared for Mega Metagross's immense power and Tough Claws ability. Your Pokémon should be at least Level 70. This is the ultimate test of your team's strength and type coverage. Focus on exploiting weaknesses and managing your healing items carefully.
