Suggested tactics is:

1. Take all south settlements
2. Advance from there.
3. Avoid Attacking the town at all costs
4. Clear all the way to the ranch.
5. Get the MCV, Follow the dried sewage river behind the town to avoid the town area.
6. Attack the the GRM command post. (Use gunship strike to clear out helicopter and tanks first)...enemy reinforcements are down after it's destroyed.
7. Now Surround the town and use paladin and heavy armor. Classic siege tactic
8. Tactical usage of airstrike is necessary to take the town.


NOTE: The more settlement you take, the faster the money regenerates. This mission is designed for tactical thinking. I give you chance to purchase it, but it does not always means you need it now. Airstrike are needed for most of the town. Take command and call in units as you seem fit based on remaining resources. I tested it, I won it on hard. So it's not impossible.