Westerado: Double Barreled

This is awarded for unlocking the non-standard ending. Technically you can do it within minutes on a new save, but having access to better weapons (dual pistols or rifle) would certainly help with your odds of success.

Technically you can begin the wave mode that leads to this ending in either Santa Anna or Clintsville, but I'd definitely suggest Clintsville. The town is larger and it's easier to run in a large circle when you need to reload. While funneling most extras is easier in the south of Santa Anna, I'd always die when the scorpions turned up.

To trigger the wave mode, just start killing NPCs in town. Eventually the Director will show up and claim you're costing him too much money and ask the extras to kill you.

The first wave is pistols, which shouldn't be too hard to clear. For the first few waves I'd suggest staying on the east side of town to funnel most of your enemies to you.

The second wave is shotguns, again it shouldn't prove too hard.

Third is Indians with tomahawks, probably the easiest wave of the whole bunch.

Fourth is Scorpion Handlers, which is by far the worst. The scorpions will take a hat the moment they touch you and are almost as fast as you are. The trick here is to realise that you're not actually trying to kill the scorpions individually (thank goodness), just their handler in black. When you kill a handler, the three scorpions that spawned with him disappear automatically. Just keep doing laps of the saloon until you can isolate and handler and pick them off very carefully.

The fifth wave is Coyote Tamers. Again, stay on the move and pick targets off one at a time. The coyotes do need to be killed individually too, so don't only aim for their handler.

Finally are the Rangers, who while definitely mighty, are not the most powerful wave. There will be five to begin with, followed by two, each with a different weapon.

Once you've killed the Rangers the Director will finally appear and ask you to put him out of his misery since you ruined the movie. Shoot him and watch the game's quite meta ending to unlock this achievement. Note that this won't give you any "finish the game with..." achievements along with it.