Instructions
Travel to Route 16, 17 or 18 at any time of day. Some Pokemon are more likely to show up at certain times of the day, but your chances of encountering a wild Slugma stay the same through morning, noon and night.
Walk around any grassy areas of these three routes until you encounter a Pokemon. As with any Pokemon you are trying to catch in the wild, you might not find a Slugma during your first encounter. If you encounter a different Pokemon, finish the battle or run away and continue walking around until you encounter a Slugma.
Weaken the Slugma you encounter with attacks from your Pokemon. As a Fire-type Pokemon, Slugma is weak against Water/Ice attacks. However, if your Pokemon are high level, you may want to stick with basic physical attacks so that you don't kill the Slugma. When the Slugma's HP is in the red, throw a Pokeball to catch it.