I remember this hard mission. I used a little trick to solve these hard missions. I know, I know
but C&C 1 had this gun-turret-trick. All you have to do is to sell a turret and when it's going to disappear klick it again and hit the "S" key to stop the selling. The turret will not disappear but you'll have 4 Minigunners for free. You can replicate this over and over until you have grand armys of minigunners. You can do this trick also with sam-sites, obelisks and nod-turrets ... I think with every building.
It's kind of cheating though but sometimes it really helps to get over the first frustrating minutes of a mission.