Gun

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Utilizing the cheat as mentioned in the Achievement Guide for Insane Difficulty Complete, this achievement can be unlocked by only needing to complete the final checkpoint. Just follow the steps below:

1. Start a new game.
2. Activate the cheat using the instructions mentioned in the Achievement Guide for Insane Difficulty Complete.
3. Pause the game and select the 'Gun' option, followed by the 'Missions' option, followed by the 'Main' option, then select the 'Ambush The Train' level.
4. Once the mission has loaded, go back into the pause menu, then select 'Options', then the 'Gun' option, followed by the 'Checkpoints' option and then load the last checkpoint of this mission.
5. The last checkpoint loads at the point where the train has been stopped and every member of Magruder's men needs to be defeated. However, upon loading the last checkpoint, the game removes all your weapons in your inventory apart from dynamite.
6. So, go back into pause menu, then select 'Options', then the 'Gun' option and then press the 'Toggle Help'. This will now load all the guns and weapons you should have at this point in the game. Making killing all men on the train easy.
7. Now that you have some firepower, simply defeat every member of Magruder's men and the mission should end eventually.
8. Achievement unlocked.

For your next task, you'll be ambushing a train with a person named Clay and your good friend Port. Follow him until you reach the destination and then place the TNT barrels in the designated places. After the cut-scene, mount the horse and follow Clay to the train. Once there you'll be opposed by a few enemies, shoot them and then you'll have to rush back to the ambush area (you should have more than enough time to do so).

When you get there, get the remaining barrel and place it where it indicates, then move to a safe location and shoot the barrels when you're told to do so. Next, you'll be faced by a series of train guards, all should be clear from on the ledge, kill them all using whichever weapons you feel you want to until they stop and the mission ends.