Idk if this will work, but maybe, be an Engi, then (from another fac) get an Engi repairing an Aircraft constantly, Make the Aircraft a light fighter (Eg. Reaver,Sycthe Etc.). Drop an ammo pack on yourself and fire at the enemy aircraft for as long as you can, preferably with a low kill time weapon, such as an SMG or any other fast firing weapon in your arsenal. As you fire at the aircraft you should get "Surface-to-air" damage rewards. It's slow, I know, but you should give it a shot. (BTW, I have not tried this yet, but try it, and if you can't get STA points with small arms, get the enemy aircraft an engi in a territory with no population, stay close to an equipment terminal and use explosives to damage to aircraft, then resupply.)