Operation Flashpoint: Red River

To get this cheevo you must play a whole mission as he grenadier class.

To change class, at the main menu select class set up, RB/LB untill you get to the desired class and press the A button.

Now play a mission. The mission can be either a campaign mission or a fireteam engagement mission.

To get this a little quicker you'll want to load up Last Stand in the Fireteam Engagements mode,choose class setup and hit RB until you're Grenadier class.Once you have started the mission proceed to the main area,then turn around and run straight back the extraction point and wait for the helicopter to come and pick you up.The starting wave will probably not even started in the time you do this,its that quick.The same method can be used for all the corresponding class achievements.