After blowing up the fuel depot at the start of the game and returning to the Belle De Nuit, drop off Luc. Do not enter the Belle De Nuit. Instead, unlock all three Mayhem perks to unlock machine guns on the Aurora race car. The easiest way to unlock the last Mayhem perk is to find an open tank, hijack it, and destroy any Nazi vehicles that appear. Proceed to "The Race" where Sean is tasked with racing against Dierker in Saarbrucken. During the race, do not pass Dierker but get close enough to him to shoot his car with the machine guns on the Aurora. Continue shooting Dierker's car until it becomes damaged and the game credits appear, indicating that you have finished the game by killing Dierker. You can either wait until the credits end or skip them. After the credits, the "The Legend Begins" achievement or trophy will become unlocked, and a mission failed screen will appear, allowing you to retry the mission. Note: An open tank is located in the ruins of a building a few meters behind the church that sits at the top of the hill. The church is located behind the Belle De Nuit.
