Smash-Head is Named Enemy in Borderlands 2. He is located in the Fridge region on the Planet Pandora. You encounter him during and after the side quest “Note for Self-Person” at the Crystal Claw Pit area. You obtain this side quest from an ECHO Recorder that is dropped by a goliath inside of The Fridge. It only drops after you competed both the “Bright Lights, Flying City” and “The Cold Shoulder” quest.
You can farm Smash-Head for the Sledge’s Shotgun as he is the dedicated loot source for this item. Smash-Head’s legendary item has a 10% drop chance.