First and foremost, do NOT buy any of the level modifications via the command shuttle.
As for a gold medal. Set the difficulty to easy and it should be a walk in the park!
I build two boosts to separate the entry and exit. Three boosts to lead them up the ramp to the left. Five gun towers on top of the boosts. Two boosts to extend the loop on top of the ramp and another to block off the path and lead 'm back down. A temporal and concussion on top of the two and a gun on the path blocker.
Upgrade to red, build a couple of towers on the exit side to catch any stray aliens and that's that!