Croatia...
	Play as NOD in Skirmish mode o...
	Play as the Scrin...
	Sarajevo...

This is the mission where you don't have enough power to activate all buildings. First off, power down the Barracks so you have enough power for all ground defenses. Next, build up 10 or so Predators and destroy the War Factories of the northeastern Nod base. This will put down the enemy armor coming from that direction, so sell the Guardian Cannons and use the surplus power to activate the Barracks and the AA Guns. When you go for the reinforcements, put APCs to the roads on the western edge of the map to prevent Militant Rocket Squads from garrisoning the buildings on your way back. When the MCV reaches your base, you have essentially won the mission. Of the remaining Nod bases: the southwestern trains infantry only, so it will be of relatively little threat. The southeastern, on the other hand, attacks with aircraft and the cliff itself is:

-impossable to ground units which means Mammoths are out of the question
-heavily defended by SAM Turrets so attacking with Orcas is suicide.

Nod will build about a dozen Venoms with laser cannons which will run through Predators fairly quickly, not to mention that the base is also defended by Stealth Tanks. Your best bet is to build a Surveyor, fly it to the northern, undefended part and deploy it with a bunch of AA Guns. Next, lure out the Venoms; when they are dead, raze everything to the ground, starting with the Air Towers.

At the skirmish options menu, hold N and click you army to make NOD as selectable option.

To switch back, hold G and click your army to make GDI selectable again.

This works with the computer army as well.

To play the secret Scrin campaign, finish both the GDI and Nod campaigns. This will unlock the Scrin campaign of four missions.

Build up your base with heavy defenses. Nod attacks intensify over time and the two Mammoths will be overwhelmed by the superior Avatars quickly. For an effective defense, capture every Avatar you come across by killing it and reactivating the husk with an Engineer. I amassed about 30 then counter-attacked, destroying the Temple defenses in one go! If you need more time, kill the Obelisks at the southernmost entrance, sneak in with a Sniper Team to spot for Juggernauts firing from your base. Once the first Ion Field Distruptor is down, call in an Ion Cannon strike on the two War Factories at the south entrance to buy some time. Against infantry, use a similar tactic on the Hands of Nod at the east entrance. Both entrances have their own Air Towers, too.