If you are keen on having a time stamp on your achievement, and want it early due to your impatience lol, there is a very easy method!
1) Disconnect from the internet via the System Menu on the dashboard, and turn the date back to 1 year. Next load up Lost Planet 2.
2) Make yourself a new saved data on a configured USB or memory cartridge. Don't select your hard drive (assuming your real data is on your hard drive).
3) Once you create the new save, head back to the dashboard again and reconnect to Xbox Live.
4) Load up Lost Planet 2 again and select the memory stick with your new data.
Once on the menu screen, your achievement will immediately unlock, with a time stamp. :)
If you want to earn the 6 month Nom de Guerre for the real saved file, you can disconnect from Xbox Live and turn the date forward and load up the game. It won't corrupt the data at all.