Puffy the Lurker is the boss of The Sandwall--the fifth adventure.
Clear out the minions every time they spawn and keep your distance from the boss--using the default gun (the bee launcher isn't as good) on the boss is effective. I wouldn't suggest melee weapons on the boss because touching the boss damages you and uses heavy attacks when you're near.
Watch out for the boss' ranged attacks and body slam move.
The achievement pops after defeating the boss.