Ancient Foes is the ninth and penultimate mission of Kane's Challenge and pits the player against the Scrin in a Red Zone. The Scrin have a heavily defended base in the northwest of the map, and a secondary base in the northeast.
All three factions can complete this mission with the same general tactic, taking advantage of the weaknesses of the AI, including the fact that they do not construct Foundries, and do not Gravity Stabilizers, save sometimes later in the game, meaning the Scrin's strong air force will not be a major issue.
Step 1: Relocate and Turtle
Your MCV will start the game in the south central portion of the map, however, it is in your best interest to move it to the high ground near the blue Tiberium fields on the southwest and southeast corners of the map. Either one will is a viable option.
One you have relocated your MCV to the blue fields, get a refinery, Power plant, barracks, Command post and War factory (or faction equivalent) built. Build a few tanks for defensive purposes, and at the same time, place defenses on only to approaches to your base, which will act as choke points. For GDI and sub-factions, a watch towers and a Battle Base one each choke point should suffice, though you may want to add an extra tower and Guardian Cannon the choke point nearest to the Scrin base, which will see the most attacks. For Nod, put up a Shredder turret group and at least one, possibly two Laser turret hubs on each choke point. For the Scrin, a few Buzzer Hives and Photon Cannons on each chokepoint.
Step 2: Shut Down The Drone Platform
Once your base is secured, its time to switch from turtling to rushing, or at least tank-spamming. The goal of this next step is to destroy the Scrin's Drone platform, preferably before they construct a Rift Generator. To do this, build about 10-15 lower-tier tanks- the Scrin and Nod will need more because of their lighter Scorpions and Seekers respectively. For GDI, in addition to Predators, you should also train some Shatterers, as their sonic weapons are highly effective against the Scrin.
Once you have you attack force, go for the eastern most (right flank) of the Scrin base- the access point nearest the drone platform. You will encounter a Storm column, but you should have enough tanks to overwhelm and destroy it. Then, move in and take out the Drone platform.
If the Scrin have built a rift generator, you'll have to destroy that too. It should be near the front of the base, but you might have to take out another Storm column. This may require two waves.
Once the drone platfrom is destroyed, order any survivors to retreat. You will likely lose a lot of your forces in this, but the Scrin will be permanently crippled- they will likely have no Gravity Stablizer or Foundry, so way to build more structures.
Step 3: More Turtling and a Superweapon Strike
After you take out the drone platform, you may be able to destroy the Warp Sphere, but you will not get much further than that due to large number of storm columns and photon cannons behind it.
The Scrin will still a portal, possibly a Warp Sphere, and may have a Warp Chasm at their secondary base. They will also still have their tech buildings. They are crippled, but do not count them out yet. For this reason, build high-end base defenses- Sonic emitters, obelisks, or storm columns at the choke points. Then, build a Tech Center (or equivalent) and a superweapon. At this point, you may also want to send a Surveyor (or equivalent) to the other unclaimed blue Tiberium field on the other side of the map and build a refinery. At this point, you should have a rapid flow of Tiberium, and be able to start really spending.
Once the superweapon is up and running, start building high-end tanks/walkers/aircraft, even an Epic Unit. Then, it's a simple matter of keeping up your defenses until the timer of your superweapon timer runs out.
If you're playing as GDI or Nod, you may want to build at least two airfields/air towers and eight Firehawks/Vertigo bombers. Send these over to Scrin secondary base. First, bomb the storm column, you'll lose a couple planes, but it will take out the lone column. Then, destroy the Warp Chasm (if there is one), then the portal, and finally the refineries with follow-up strikes. If you're playing as the Scrin you can take out the secondary base with a few Devastator Warships.
Once the countdown ends, fire your superweapon on the concentration of defenses at the main base. This should also take out any remaining productions structures. This will leave you free to deploy your by now sizable army, including heavy or epic units to finish off the survivors.