I managed to get this achievement in the E.F.F. campaign on MISSION 01. This mission is a soldier only mission as there are no vehicles or MS' to enter. The class you spawn as is the Assault soldier as well.
This is unmissable as you have to clear out enemy soldiers to advance with the mission anyway.
I'd recommend clicking in the Right Analogue Stick to zoom in your weapon. Remember to run back to the Health and Ammo crates if you are low on supplies or need healing.
Please comment if you vote negatively. I will edit my solution to make it better.