Pretty easy to unlock, just purchase one of Porkrind's new weapons for 4 coins (either Crackshot, Converge, or Twist Up, located on Isle IV) and make sure to get the killing blow on a boss with one of those weapons. This can be done on Simple mode, and it does not have to be a DLC boss. So for example, you could go back to Botanic Panic
I recommend Crackshot due to how op it is.