High Noon

Needs an enemy who's also carrying a firearm, shot dead with your own gun - so it's gated on the late-game once hand cannons are about. The easiest setups are quests where bandits or soldiers wield guns (the Dry Devil stretch is a reliable spot). Keep a loaded hand cannon ready, single out an armed foe and put the shot in. Not missable - guns and gun-toting enemies are around plenty once you're that far.