||situation: It is very annoying for GDI having a NOD opponent who uses Apaches: Apaches kill infantry, Harvesters and important structures.
What to do? GDI has 5 different anti-air units:
- rocket soldier
He is only effective versus Apaches, if used in groups. Spread the group, because the Apache's high-speed Gatling cannon spread it's fire a little bit and would damage the whole group. Use Rocket Soldiers as backup forces in emergency situations like saving Your CY or WF. I suggest at least having 2 Barracks for speed. Attention: Rocket Soldiers can be squished!
- mammoth tank
If You have them, You're out of the woods. If not, You need the Repair Facility to build them. Mammoths take a lifetime to be built except You own 2 WFs. The Mammoth Tank is nearly undestroyable by Apaches and self-repairing up to 50% health for free. He owns not only his dual 120mm APDS for ground targets, but also his Mammoth Tusk missiles for ground-to-air fighting. Put a Mammoth with some Medium Tanks or Grenadiers in Your Tiberium field and Your Harvesters will stay alive. Using the Mammoth for anti-air defense in Your base is a good idea too.
- advanced guard tower
A great unit versus Apaches and infantry. It uses Tomahawk missiles. But it requires the Communications Center and an extra Power Plant. The disadvantage is that You cannot move a tower if needed elsewhere. You have to build a new one. Expensive.
- orca VTOL assault craft
His Dragon TOW rockets can hit Apaches and other helis air-to-air. Attack some NOD-Apaches on their heli pads. If they take off to evade, no problem. The GDI-Orcas follow them and fire at the flying targets. Apaches cannot do the same, because they have only air-to-ground capability. If NOD has no anti-air units this tactic is very annoying. Make sure to have Your Orcas number-grouped with "control-number". So You can select them in the air and get them back with "s" or by directing them elsewhere case the Apaches circle over anti-air-capable-units. The Orca has the advantage, that the Apaches have a bad chance to reload. If the Apaches land You can target them! That renders the Apaches nearly useless. Furthermore NOD has to build some anti-air units herself. Nice waste of money. Money, NOD cannot spend on more Apaches. If NOD don't build anti-air units the Apaches will be toasted by the Orcas.
- mobile rocket launch system
This unit has the disadvantage of bad armor and no short range capability. But it's not very expensive and good versus everything. And You need an Advanced Communications Center to be able to build that MRLS.
I suggest the Orca as the primary anti-Apache unit. Let the Rocket Soldiers be backup forces in emergency. You will seldom have the chance to build Mammoth Tanks. Advanced Guard Towers are very local. The MRLS is nearly unavailable because of the Advanced Communications Center.
By the way: You can use an Ion strike to destroy an Apache on his heli pad.