Use the skynet assault guns alternative fire to kill any enemy with 1-2 hits, it will even take out the medium tank with minimum ammo loss, and alot of t-900's use it, the only downside is it isnt as accurate with strafing or moving targets as primary fire, but the energy cost will be the same usually, so dont worry about other, more powerful weapons unless there is a group of enemies, but then use the tech-com RPG(aim at center of group nfor optimal damage).