
1. Find the NVA invasion route.
2. Place a green beret so that it can see up till the map edge.
3. Place an outer outpost nearby.
4. Place a strong hitting unit (a tank or cobra) within range.
5. Place an artillery unit in the outer outpost.
Now, once the NVA appears, hit them with the tank/cobra. If they survive, they will retreat to the map edge, so they will still be in sight range of your green beret. You can then pound them with artillery. If they still survive, you send in the airforce or perhaps an infantry drop. Chances that NVA will survive till next turn is close to none. Repeat again once new NVA guys come round.
This is the originsof my "let'save Laos/Cambodia border" hexes suggestion. Right now NVA that entered the map have nowhere to run unlike history where they retreated over the border ALL THE TIME and annoyed Westmoreland immensely. This was really what caused the US military to request an escalation of the conflict and expandng US presence to LAos and Cambodia.