If you GDI, then try APC's with Rockets(around 15-35), have the APCs target the PAC (Planetary Assault Carrier) itself, and the rocket men will shoot the planes coming from it, that way you dont take much damage (maybe lose 10 APCs). If thats not working then just try Mammoths or AA's.
As NOD, the Stealth tanks can do a lot, and if you defend with a lot of SAMS then they wont hurt you at all. The sams are much faster then the PAC Planes, and shoot them out quickly, leaving the plane without any defence but an Ion Storm. You can take out 15 PACs with 10 SAMS (Had it happen to me yesterday!)
As Scrin just use the cheap tanks, the first 2 you can construct are strong against either Infantry and Air, or Tanks and Air. So not much to worry about, and if your still havin trouble try building a PAC of your own or some Storm Coloums.