This may not be the best way, but it worked for me.
First move at least one of your medics far to the rear of your forces, out of harms way. Then engage the Gungan Armies with your forces. Use darth Maul on the sheild generator units, and as he gets damage, pull him to the rear to be healed then send him back in. All of my forces were destroyed except darth Maul. He had to defeat quite a few of the Gungans on his own. To make it easier on him after all my other forces were deafeated, each time I returned him to the battle, as soon as a few gungans were in sight and they started after darth maul, I would bring darth Maul bak a litle bit so he would not have too many gungans to fight at once. Though he is quite powerful. eventually the gungans ran out of forces, and it just became a matter of time. I hope this helps.