Instructions
Travel to Route 16, Route 17 or Route 18 using your preferred method of travel.
Walk around in the grass to trigger a random Pokemon encounter. Slugma has a 5 percent encounter ratio at any time of the day, so you will likely encounter several Pokemon before finding a Slugma.
Attack Slugma using your strongest Water-Type, Ground-Type and Rock-Type attacks to lower its health points. Continue attacking Slugma until you think it is weak enough to capture.
Navigate your Item Menu and select a PokeBall to attempt a capture. If Slugma has not been weakened enough, the capture will fail and you will need to attack it some more before trying again.