#3 Pew Pew Pew
Play as one of characters with a Western / Cowboy background like Prickles, Boomer or Rattles. Play this character for a while and after a while the Shooty Skies mystery character Pew Pew Pew will be unlocked, it’s a gun fitting to the western environment. The exact condition is unknown, but this character is rather easy to unlock, just keep playing for 5-15 minutes.