Mission Briefing
Narration: The Arm had been driven back high into the hills of Empyrrean. There, the troops massed for a final, desperate stand. No quarter would be asked, none given. The Core intended to destroy the Arm completely, finally ending thousands of years of war. The Arm would fight to the last.
Intell-tap The final mission. Arm has been pushed into the hills for one last stand. The Arm Commander will be with his units. Destroy him as well.
Destroy Arm
DO SO NOW
Details
Starting units: Commander, 2 Thuds, 2 Pyros, Can
Pull back to the southwest corner and find some metal deposits to start resource collection. Once resources are coming in, build a KBot Lab, Vehicle Plant, and Aircraft Plant right away. Each plant should immediately build a Construction unit, followed by KBots, tanks, and fighters. Build Crashers and Slashers first for anti-air capabilities. Start building the Advanced Plants while these units are coming online.
Crank out Cans, Pyros, Mobile Artillery, Reapers, Diplomats, Vamps, Rapiers, and Hurricanes. Put the level 1 units up at the front of your lines, and back them up with the higher level units. Spread east along the southern edge of the map and maintain troop levels while gaining new sources of metal and bringing more energy collectors/generators online.
Once you've spread across the southern edge of the map and built up enough units for an assault, start moving slowly north. Keep the Diplomats at the back of the force, with Deleters and Informers mixed into the group to keep targets available and the force hidden from Arm radar. Plenty of KBots and Vehicles should be held back to odefend the weapons, with the frontal force drawing Arm out from their base. As you get within range of the pass, send a couple of KBots ahead to identify targets, there should be some defensive structures near the pass that would be best taken out as soon as possible.
Slam the whole line of the pass and the surrounding area with fire from the Diplomats before moving in. With some luck you may even take down the Arm Commander (though you still have to remove the rest of the Arm troops). As you move into the hills, make sure you have a strong air force to cover your ground forces in the approach. Bring forward some construction units to place defensive structures along the hills to prevent Arm forces from pushing back through the line or slipping past to attack the base.
Make sure the factories have a deep queue of units in production and send your air units in to clear the path northward once the hills open up. Keep units moving to the front line and keep up heavy bombardment just ahead of your units. A Bertha on the ridge will cause trouble at this point, and you'll have to take it down quickly with bombers and Diplomats. This is where the advance slows to a crawl, but long-range weapons and bombers will win out if you can maintain production. If you have a lot of extra tanks moving to the front, push them forward into the frey. Eventually anything that isn't long-range or a bomber becomes extra scrap to throw against the enemy in hopes of reducing their numbers and pinning them down. It could take a while, but as long as the units keep coming from the Core factories, this is where Arm is annihilated.
Alternate strategy
Although this may seem like you are completely out in the open and have to maintain a huge defensive line, the reality is that the enemy really only focuses its ground effort at the center of your base. Get started right away with 2 metal extractors/2 solar panels and then build an aircraft plant. Your construction aircraft should start building 2 punishers (have your commander finish them) about 1/3 up the map and at the center; afterwards, have the aircraft secure all the metal deposits below this line. Build also a Vehicle plant so that you can get your C-Vehicle building pulverizers all over your base (Arm will at some point send a huge phoenix fleet to destroy your mines/power plants, scary?). Afterwards, have your vehicle plant be constantly pumping out slashers and use the 'move' command on the structure to make them rally in front of the punishers (to draw enemy fire and spot for them). At this time the enemy should already be sending a stream of units, but your defenses should be more than sufficient provided you have a steady flow of slashers. If your radar detects stationary red blips near your location, send a task force of slashers to destroy them (they are most likely undefended mines/construction workers)
Your aircraft should be done building metal extractors so now is time for the fun part: build two advanced aircraft plants, one at each edge of the map, parallel to the line of punishers. The sooner you can do this the better (use the commander to speed up production if necessary). Once this is finished (and you have your advanced C-aircraft out), set the rally point for each plant to 'patrol' at the other edge of the map (right side for the left building, and vice versa), but slightly more forward, so that the airplanes will cross somewhere at the front of the punishers in 'X' formation. Not only will this help spot for the punishers and take care of stray units, it will essentially give you air superiority (Vamps are the best unit for taking out enemy aircraft). Your fighters are always in the air as soon as they are built, so 95% of the time they will take care of enemies they see themselves, thus giving you total defensive coverage. Of course, don't be stupid and put the rally point deep in the enemy base right away. Then, queue up a massive amount of vamps on both plants. Oh, and build a couple of pulverizers in front of the plants in case any aircraft/stray units decide to rain on your parade.
I assume you are upgrading to Moho Mines and fusion plants in the meanwhile; you know the drill. Build the other advanced plants and kbot plants if you want (if only for the extra cannon fodder to accompany your slashers, and the construction units that grant slight metal/energy bonuses). You should get an intimidator cannon running at the moment (set to fire at will). As soon as your 'X' patrol of fighters is uncontested, feel free to inch up the rally points so that the vamps will occasionally run into the enemy. You want to maintain some tension so that your active stockpile of vamps are doing some work, but not too deep into enemy territory that they don't have a chance to survive and build up into a large airforce.
Eventually the 'X' sweep will scissor into their base (most of their AA should be gone by now, automatically taken out by stray vamps). Now is the perfect time to send in your slasher army (with whatever else you built for defense, mine was Goliaths/Diplomats). They should be able to wipe the base with its commander no problem. As for the Big Bertha, it may start turning toward your base once the vamps spot it, so quickly summon 10 of them to attack it directly and that will be the end of it.