This achievement can be unlocked solo without any opponents
Head into Multiplayer - Xbox Live - Race:
Custom Race: Make sure the discipline is Rally, Press
to search (If using ranked) and more than likely you will be put into a game on your own.
Head into the Voting
section and pick a vehicle and track you feel comfortable with (The slower the better in my opinion) Once the 60second voting period has ended the race will load and begin. You can do this achievement in a private match if you wish
Once the race begins take it easy, there's no rush as it doesnt matter if you win or lose (Impossible on your own anyway) Make sure to not hit anything (Including the tape barriers) the achievement will pop once you cross the finish line.
If you havnt done so already you will also unlock
The First Clean Race achievement in DiRT worth 17 pointsComplete a race without colliding with anything in any game mode.
Here's a video