Quite simply. Just load up the game on easy and beat the Boss. This Boss isn't too challenging even on Hard. Basically destroy the spawners to keep enemies away and stand on the two switches on each side of the map to spin the circle in the middle until the Boss's heart appears in the middle. Then attack the heart. Repeat until dead. If you are doing this solo you will need to run back and forth between switches as the Boss will chase you and is an instant kill.