How to deal with the Scrin
The biggest key here is finding enough money. Even on the smallest map, if you can control at least 2-3 Tiberium Fields, you should be ok.
Make sure to place a lot of anti air, all over your base. This stationary defense, combined with the mobile anti-air force provided by Attack Bikes should be enough to force the Scrin to fight on the ground where they are weaker.
The key to overcome PACs is the Attack Bike. A group of Attack Bikes will take down a PAC in no time flat. The key here is to use attack move to speed around under the carriers, hopefully avoiding a lot of damage and making it harder for the player to click on a single bike to concentrate the PAC's fire.
Go for Lasers as fast as you can. Build at least 2 Airfields spamming out venoms, while working on the Laser Upgrade. The PAC's are not good enough against the Venom's Lasers so that will shore up your defenses. If you get about 10-15 Venom's with Lasers before you get rushed by the Scrin, go attack, and continue to find new Tiberium Fields.
Annihilator Tripods deal high damage, and are good against groups as well as single targets. To take them out, you should follow the same strategy to take out a Mammoth. Then if you can get a Sabotuer inside of it, it will nicely compliment any Nod attack force.
Mastermind can mind control structures and units, turning them against you. Shredder Turrets all over the inside of your base should protect against them. When they mind control a unit, remember, that unit is still yours if you can kill the Mastermind. So don't waste time fighting yourself, and chase it down with one of the Raider Buggies you should have lying around.
You can also try outpost rush strategy on the Scrin. To do this you should build a crane, first. Build an emissary while building the crane. When your crane is finished, start building a power plant and refinery immediately. Also send out your emissary to the enemy's base. When the power plant is done, start the construction of another, and with the shredder turrets unlocked immediately, then build a hand of nod. When the emissary arrives at the enemy base, deploy it immediately. If the enemy attacks the emissary with gunner infantry (the anti-infantry) repair it. If enemy attacks emissary with rockets, sell the emissary and proceed with a different strategy
When refinery is done, deploy it at my main base. When Emissary is deployed, set up a hand of nod and two shredder turrets immediately. If the enemy sets up anti-infantry defense, build a laser turret, deploy it, and have them attack the power plant or anti infantry. Build either rocket soldiers or engineers at your hand of nod while constructing another refinery. Start attacking or capturing the enemy base and deploy the refinery also. If the enemies base is decimated, but he does not lose, send out assorted soldiers to each of the other tiberium fields in the map immediately, if you have not yet deviated and taken them.
