Defeat BoJohn at the end of the cemetery in the suburbs.
BoJohn has a megaphone that he uses to summon underlings to fight alongside him. Take it out first to stop the flow of helpers. Then focus your attacks on him. He will fall in short order.
I recommend the robot and statue of liberty costumes along with the pumpkin slime stamp. Splash damage from the pumpkin slime should do a decent amount of damage to BoJohn over the course of the battle as you attack his megaphone and underlings.
Once he is defeated for the first time, the achievement will pop.