World at Arms

This is very easy and can be done at any time. The earliest time to get this is right at the start of your game. After your first fight is complete, go to quests and select “Pre-emptive strike”. Use just your humvees for this battle as they are your strongest units, then when the fight starts, hit the bomb icon at the bottom of the screen and select the missile. You will probably win many battles without losing units as you play, but you may as well knock it out early.