I'm no good at planetary battles

[Silverman]

Hard to say.
Agro-Intestinal Complex would be a fun one. This, because you have only one way to defend at the beginning. Also you can practice fighting with a robot. Tips for this:
Never ever get in range of the enemy robots, as they'll blast you. Normally they should be many more than you (Xv1). Instead use the range of the Rocket Launchers and walk backwards while shooting them.
Conquering factories is easy as well. Just go near enough to a turret to get a red reticle and shoot as hell. The enemy turret won't shoot, so is an easy kill. Still watch out for enemy robots.

EDIT: Try doing the test-mission building only one robot. This is to train yourself fighting with a robot. I'd take the bot with 4 rockets and the other stuff. Some tips:
Before you attack, be sure, you can run backwards shooting like hell. This is to avoid getting hit by the bots. Also be sure, you don't get too near to the enemies. Because they are walking, you don't need the red recticle but can shoot when it's near to the red area. Always run backwards when attacking the bots, and of course don't try running through the whole mission at once, take short breaks and plan the next attack.