This achievement is not hard because it's just a matter of holding out until you are ordered to evacuate.
Don't lay ANY mines in this mission.
- Put your shock troops and conscripts next to the two sandbags and the wall in the middle just outside your base. Garrison the HQ and the watch tower with infantry, and put both engineers (with flamethrowers) in the sandbags in the middle of the base.
Now all you have to do is wait, while reinforcing shock troops and engineers with conscripts. Keep producing conscripts and calling in reinforcements. When you are ordered to evacuate to the bridge, use the "retreat" command on all troops.