That final boss was too difficult. Press up with the triangle, then get the circle to roll under it, then go into the square shape at the end, and finally there is no final boss. ;)
Crusaders become available to use as you earn more Gold. There are 20 base Crusaders to use, and you can unlock more spaces and variety, but how many you can put in the formation varies by the campaign and the objective.
 "STUG"){kind=icon}
 "circloO 2"){kind=icon}
 "The Man Who Can't Jump"){kind=icon}
 "Cyclomaniacs Epic"){kind=icon}
 "They Took Our Candy"){kind=icon}
 "Revenge of Dog"){kind=icon}
 "Pixelvader"){kind=icon}
 "Snakes On A Cartesian Plane"){kind=icon}
 "DJManiax"){kind=icon}
 "Shift"){kind=icon}
 "Bot Arena 3"){kind=icon}
 "Cubito Mayhem"){kind=icon}
 "I wish I were the Moon"){kind=icon}
 "Crusaders of the Lost Idols"){kind=icon}
 "Road of the Dead 2"){kind=icon}
 "Dynamic Systems"){kind=icon}
 "Song for a bird"){kind=icon}
 "SUPERHOTline Miami Deluxe"){kind=icon}
 "Kaleidoscope Reef"){kind=icon}
 "Unpuzzle"){kind=icon}
Indeed, you need a mouse.