This achievement is easily attained by playing through the story line and should unlock fairly quickly in the game. Try to aim for the head of the enemy soldiers as they pop up and the action camera will enter slow mo for a wonderful "Pink Mist" moment.
The action camera will also activate for explosions of any type ( i.e. Bazooka, Grenade, etc. ) The action camera moments are cumulative and are retained even if you are killed in action.