For this you need to complete the 12th mission of the campaign. In this mission, you alternate between controlling Frost and an AC130 gunner. Simply play though the mission, completing the actions they ask of you. You can play on recruit to get this achievement, as it can be obtained on any difficulty. You get various checkpoints throughout the mission, so I reccommend taking the mission slowly and pushing through until you get a checkpoint. Like every other mission, if you die you just get sent back to the last checkpoint.
This achievement is story related, so cannot be missed by natural progression in the game
The achievement unlocks at the end of the level, when in the loading screen for the next level.
This video might help:
It is not my video, credit goes to: theRatBrad