It doesn't matter what faction you select but the Imperials usually win. Two player works really well. Spawn as a Marine and get to your transport ship and try to pick up a few of your fellow units. Fly over into the hangar of your opposing force and land. Get out and people will start spawning from it. Use your blaster rifle to clear anyone in the hangar before heading into the doors. Use your rifle again to eliminate the auto turrets. There is a glitch where if you go in the left door and stand near the wall facng the control room you can get health from the medical droid on the other side of the wall. Back on topic, sprint to the shield generator and destroy it with missiles and thermal detonators but it helps with two people and gets it done way faster. Blow everything up, but watch out for those pesky auto turrets and often some enemy will spawn right in front of you so keep your eyes peeled. Once everything inside is blown up, leave your transport ship there and hijack the enemy ships so you can destroy the things on the outside of the ship.