For me, the following combo of units and steps worked well to get thru the campaign:
Crawler / Shield / Crawler / Crawler / Shield / Support
Start with Crawler / Shield / Crawler As you begin to earn money, purchase upgrades in this order:
Upgrade first crawler
Upgrade second crawler
Upgrade first crawler to max
Purchase third crawler
Upgrade second crawler to max
Upgrade shield once
Purchase second shield
Purchase support
As you move thru the map, keep your units protected via smokescreen and decoys, particularly with the larger tower units. Keep everyone repaired as needed, try to deploy repair kits just a tad bit ahead of the unit so the repair runs as long as possible.
If you are on an escort mission, follow the path that has the yellow circles around the red towers. This is the path that your escorted unit will be taking and it needs to be cleared out. You may need to make your units go in a circle to allow the escort unit to catch up with you.
For the mission where you need to keep bridges from being destroyed, look for white Airstrike icons behind the walls next to the sentry towers. You can destroy those towers via airstrikes without being seen, which is a big help.