Are you stuck on the easy campaign or the hard campaign???
Well i've done both of them easily, but heres what you do:
Use Rachel on the Bottom screen and a Strong CO of your choice.
Rachel needs to have skills that will make units CHEAP to deploy, and your other CO needs to have something that will make the untis stronger in defence and offence (eg. Slamguard? Brawler? etc...)
You do know that you can also control the top screen on this level, right? Then DO SO. What you need to do on the top screen, is just to try to be VERY defensive, and if you can, try to build and send few Anti-airs and tanks. You should be find with those...
The bottom screen is the harder part. What you do, is try to capture the base on the left hand side of the screen and the airport on the right-bottom side of the screen, and all the cities that surrounds the volcano just outside the mountain range. you really need to use your APCs well here, bcuz there's a time limit and also APCs use their fuel quickly...
Once you've done capturing, move into attack. Place your Rockets and Artilleries so that they're killing the enemy units which are going towards your base/ airport. Build few Md or Neo tanks on the base and bombers on the airport to push the blackhole back.
Also u could build b.copters to aid the Mechs and infantries to kill the Neos& Mds in the middle isle and capture the cities.
Also, with the base on the left, u will need to build STRONG tanks to defend there and maybe a rocket from the middle-part of the map to aid it. This goes same with the airport but with bombers or b.copters (make sure that you kill the anti-airs first!!)