Slugma locations — where to catch

In Kitakami (Scarlet & Violet – The Teal Mask)

• Infernal Pass — Walk, Static Encounter
• Oni Mountain — Walk
• Paradise Barrens — Walk

In Sinnoh (Diamond, Pearl & Platinum)

• Stark Mountain — Walk

In Johto & Kanto (HeartGold & SoulSilver)

• Route 16 — Walk
• Route 17 — Walk
• Route 18 — Walk
• Violet City — Gift

In Hoenn (Ruby, Sapphire & Emerald)

• Fiery Path — Walk
• Route 113 — Walk

In Sevii Islands (FireRed & LeafGreen)

• Mt. Ember — Rock Smash, Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Route 16 — Walk
• Route 17 — Walk
• Route 18 — Walk

Other games (5)

• Brilliant Diamond / Shining Pearl (Gen 8): Stark Mountain
• Omega Ruby / Alpha Sapphire (Gen 6): Fiery Path
• X / Y (Gen 6): Friend Safari, Kalos Route 13
• HeartGold / SoulSilver (Gen 4): Primo
• Colosseum (Gen 3): Pyrite Town

Slugma — full Pokédex entry