Jeb is a Jesus-like looking hitman who comes from the Madness Combat universe, like many other characters from that series, and you and BF will now attempt to defeat him not with guns, but with good rhythm, challenging him on the following songs:
Use music not guns to defeat Jeb!
After you make your choice between the story mode and free play mode, give it your best to reach the end of the songs by playing all their notes correctly according to the charts, the only way to win.
This means that when you see arrow symbols matching above BF's head, you need to press the same arrow keys, but be careful not to miss doing it multiple times in a row, or you lose and have to start again from scratch. Good luck!