Ok this is quite old... but I have only gotten to this level just now. I haven't actually started it yet.
I found this site very useful. http://www.funkylydia.com/perimeter/levels2.html

Wow, I just finished this level, and yup it's tough. I would say in addition to that site, do these things:
- Create another command center (4 total) and another soldier lab as well as a tech lab. Alternate labs to keep Piercers coming. You want one for the first frame, and one for each lab after that. You can also join squads if you need a few extra of another unit.
- Keep about 6 R-Projectors on hand, running around defending against those nasty scum splitters
- Create a transmitter on the front of the central section, you want to be able to take over the front of his network as soon as you create a break in it. It seems like your frame won't cross the mountains until the portal is captured.
And like most of the other levels, keep at it! It seems like it takes me 5-10 tries to finish some of these levels.

The strategy I used was similar to BoxOfSnoo's solution. Since you start with the units necessary to make a Piercer and an Unseen, I keep those units supplied with the same configuration. The Unseen lets your piercers get close enough to the enemy Frame to disconnect it, and you want to do that as much as possible in order to prevent it from charging on the surface and popping through the Omega. Secondly I found that trying to get my own frame through the little pass in the mountains right between the two bases was just about impossible. The height of the mountains makes it so that the Frame can't pass. What I had to do was build a network of energy cores and transmitters around the southern end of the mountain range. After my frame was charged I got it truckin' down that route at just as fast as those energy cores were complete. The 4 command centers help since it's advantageous to have 4 piercers to take out the enemy's vital infrastructure so your frame won't get hammered on arrival and you can also capture the Omega. I built the flying rocket units to respond to any threat from the enemy and turrets around the entire base to hold them off so they don't mess with my ability to get on top of the situation. It sucks if you're not paying attention to your transmitters and they get knocked out by those tanks, then your whole base is out of power and your Piercers vanish.