This one can be tough only if you let it be. The enemy tank general is at the choke point, preventing you from getting through or taking the town. All you have to do really is wait a certain amount of time, keep your generals alive and he will move out himself.
When he lands on the secondary objective, you will get more reinforcements. At that point it’s a race for the clock, beat the general and hold the town before the turns run out.
Hope this helps a bit, good luck!