The description is misleading - it's actually 50 outlaws ('L' in the achievement icon is the roman numeral for 50). Also the outlaws that are replacements for the preacher aren't counted (i.e. those in the mini levels after The Mines and The Town).
You can find where the outlaws are going to be located for your run in the bar - the 3 wanted posters on the back right of the north wall will list them in the level order.
I initially used them to plan my route through the dungeon, but in the end skipped those in the Swamp/Canyons/Graveyard as the added difficulty of those levels caused too much frustration when trying to get to the final boss.
When trying to farm them, I'd check the posters in the bar and if The Crypts wasn't listed, I'd quit and start a new run.
I was tracking my counts and it unlocked for me at 55. It's possible I miscounted somewhere or there's a bug in the game.