Hint: Jet mode:
The weapons in Jet mode are considerably less effective that the weapons in Walker mode. Couple this with the fact that Jet mode does not allow the Shield to generate. Only use Jet mode when you need to blow something up or kill a Fighter (or any other air units such as bombers, landers, etc.) in the air. Stay on your side of the battle, as it will be nice to have a helping hand when things get rough. Try not to use all your energy while in Jet mode; when you land you will have no shield. Repair Pads are your friends -- use them.