Okay, I think this is the solution:
Choose Grit, Andy, Eagle
Grit and Andy need to hold their ground and not worry about advancing - just make sure the enemy doesn't capture your HQ (they will!) or destroy all of your units. If you lose one CO, you automatically lose the mission.
With Eagle, focus on creating bombers; move them up and blast through the pipe, then move them up behind the one cannon and blast through the double-pipe wall there and into the main enemy stronghold. They can own that laser cannon pretty fast, but keep on eye on what day it is; it fires every seven days and it would suck if it zapped your bombers.