This one is a long grind, but easy if you get a decent rocket launcher and homing missile launcher (w/multiple shots) and do the last mission (On the easiest difficulty).
Destroy all troop ships and Hectors, plus the mother ship cannon and also the spinning plates that shoot you.
Then you can stand directly under the mother ship and rubber band the fire button on the missile launcher to kill a constant stream of gunships. Better ranged missile launchers are a plus here.
Personally I rubber banded the right stick also to have my character spinning (and avoiding potential LCD burn overnight).