Just did another run through on the campaign, and wanted to share a few tactics that can help people out.
Some background: I'm in my 30's and have played this game since the release of the demo. I also wrote the only guide on GameFAQs. I've also been in the military for over a decade, so that has also helped as far as tactics and resource/task management.
My primary philosophy is this: I am commanding nonliving machines. That means that any and all losses are acceptable, so my tactics tend to generate an extremely high casualty rate for success through attrition. These units are expendable, the ultimate goal is mission accomplishment. This makes playing this game fairly easy.
Here's a few general tips:
Pushing The Front - Once you have established a solid source of income through mining, start spamming military units. I prefer Wraith Plasma Reapers and the two lowest tiers of Knights. All other units take too long to produce and are prone to dumb accidents or losses through heavy duty enemy machines like Gorillas and Eradicators. Set up a fireteam of 3-5 reapers and have them follow the reaper that you're controlling in first-person while you run around and shoot things up. Be sure to set their initiative to high so that they're aggressive and kill everything. While you're doing this, change the military unit factory to assemble at or ahead of the front line that you're creating. That way this will produce a solid stream of reinforcements to replace the units killed in your squad.
Strike Early - Often times in campaign the enemy has not set up their base properly upon your arrival. Set up some basic mining/production while you punish them for this lapse of judgement. Prioritize destroying factories, turrets, and military machines.
Factory Denial - Place a machine on the spawn pad of a factory and have them stand ground. This will prevent enemy units from spawning while other units destroy the factory.
First Person Special Ops - Get into first person and weasel your way into base. Then just go around and kill and destroy everything you can before you're overrun/trapped by enemy machines. You can cause a significant amount of damage this way.
Here's a few mission-specific tips that I've observed over the years to be especially challenging:
Eliminate Enemy Presence - Spam reapers and use the tactics outlined above.
Research Facility - Same as above. Use first person to run ahead of the main attack group to distract and kill eradicators. Build the eradicator only to accomplish the mission, you should be able to destroy the enemy base by the time it's manufactured.
Military Fortress - Build your base in the southwest corner of the map. Set up a defensive line to the north. Press to the east and then north, attacking the main base from the east, inwards to the west. This will be helpful due to destroying the heavy factory first, then mediums, and from there it's cleanup against light units.
Take the Pass - This is my second favorite mission. Set up your mining as well as more advanced mining units to the north of your position across the water (you'll have to go around to the east, then north, then west). Be sure to be on your research and factory game to accomplish this. Spam reapers and bottleneck the enemy in the pass while you establish your base. There will be an air attack, so be ready for that. Once that's over, you should have established a good base to begin the vicious press through the pass. Expect major casualties. Once you break through the pass to the other side, focus on eliminating factories to relieve pressure and only engaging enemy units as they come to you. Do not go out of your way to engage far away units, as the time used to engage them will allow the enemy to produce more units using the factories you didn't destroy.
Reclamation of Talonthia 8 - Forget the little base you start with. Get research done, spam reapers, and press north into the city. You'll begin to regain control of some units and factories. Use the civilian factories to produce constructors to build military factories to spam units. You will regain control of a robust mining system, so take advantage of the pause button to assess what you have, and build mining transport to exploit it. Destroy any and all enemy military factories you find in the city. There will be a late-game air attack from the northeast against your mining assets, so throw in a few anti-air turrets on the east side of the city and watch them get shot down before they're even close. The route to the enemy is bottlenecked at the west side of the city, which will be the front. Be extremely cautious approaching the enemy pod, as once you near the north exit of the city, the enemy will use the ion cannon. I usually throw a few expendable units to bait the ion attack and then move in the main attack group for the press into the base. Prioritize denying factory spawns while you destroy the pod. People make this mission out to be way harder than it actually is.
Behind Enemy Lines - My favorite mission. Here's quick and easy way: defend and focus on researching/producing an Eagle APC. Go into first person and fly out to the commandant and pick him up, and fly him back. You'll take fire but the Eagle APC can handle it. The FUN way to do this mission is to actually to a frontal assault. Use the tactics outlined above to push the front. There will be enormous casualties. Have the commandant go as far south as possible, and then walk him up the hill, facing northeast. Put him into high initiative, and have him stand ground. He'll kill anything that gets close to him on his own.
Finger of God - Another fun level. Basic tactics outlined above. You have the Ion Cannon, so use it as soon as it's available. The objective is to destroy the enemy pod, so technically you can win the level doing just that, but it's not fun that way. There is a sneak attack a few minutes into the level from an APC dropping a few wraith reapers into the back of your base. Don't waste resources building an anti-air turret, just have a few units ready.
Protect The Controller - Stupid easy level. Spam building nukes, and send all units to the northeast corner of the map to destroy the factories, then press west to destroy those factories. Send builders to capture the pods. Once the north half is secure, move units to the south. By then your nukes and ion cannons will be ready to strike. Use your remaining units to mop up. I'm considering doing this level again using only reapers after a ten minute delay for a challenge.
Disable Relay Station Force Shield - Hate this mission. First thing, get your wasps to destroy the enemy air factory to the North-North-West. Keep them moving as the long range missiles will destroy them. Once that is destroyed, press north and destroy the final air factory that's guarded by anti-air. If you approach from the southeast, there is only one turret that can attack you, and if you get the angle right, it will shoot the little hill in front of it and destroy itself or at least damage itself. From there kill it, and then deny the factory spawn while you attack it. For resources, the best asteroids are to the North-North-West of the red ship, and to the southeast of the red ship. There will be an air attack, so be ready for that. Once that air attack is over, there will be no more due to you destroying the air factories. Otherwise you'll be constantly harassed by them. Once that's done and you have income, load up the APC's full of reapers and have them land on the southwest pad, where there is a turret. Destroy the turret, send the APC's back for reinforcements, and have the reapers establish a safe landing site. The reinforcements should have a builder so you can create a factory to produce more reapers. From there it's success through attrition.
Final Assault - Fairly easy mission. Stabilize the area by destroying the two long-range missiles to the east. Build some rudimentary mining, and a medium military factory so that you can create the Judas. Use the grunts to rush the south, focusing on attacking the long range turret. Use the Gorilla in first person immediately after them to wipe out the military factories to the south. Once the Judas is made, use it to press east and then south, using capture units to destroy factories. You must be quick in this mission, otherwise you'll quickly get overrun and frequently nuked.
The Controller - Judas Warlord is kind. Capture machines, use them for factory denial. Capture a machine and have it destroy the factory it came from. Press to the north and move in a counterclockwise fashion. Save the game a lot.
Cheap Tricks (I use these to skip boring/tedious levels):
Fire Turret Exploit - Build a fire turret near a Gorilla or eradicator. They will attack the turret regardless of what's around it, meaning that you can easily have the enemy destroy its own structures. Because it's small, sometimes the units will miss. This can also be used to destroy machines clustered near the gorilla/eradicator.
Inside Job - Pods and smelters have interiors that units can enter. You can get a grunt into an interior using first person and have them shoot from the inside until the structure explodes. You must stay in first person, because if you leave first person and command the unit to attack, it will actually move to make space for an enemy unit that will kill it due to AI limitations.
**Let me know what you think and how these tips have helped you! **