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.