Colt Canyon

This unlocked for me in area 2-2B on the run back after rescuing my partner. Prior to that, I had been clearing most enemies off the levels (but not making sure to kill literally every one), so there is plenty of leeway here if you don't kill every enemy in a run. In my looped run, I killed 1200.

Otherwise, the only strategy here is to explore most of each level, killing all enemies.