Where to catch Slugma

In Kitakami (Scarlet & Violet – The Teal Mask)

• Crystal Pool — Walk
• Infernal Pass — Walk
• Oni Mountain — Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Fiery Path — Walk
• Route 113 — Walk

In Johto & Kanto (Gold, Silver & Crystal)

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

Slugma — full Pokédex entry