In the Raid At Bakura mission, first meet the Silver Medal requirements for the mission. When done, you will get a TIE Bomber and fly to a canyon. Once in the canyon, use your lock-on missiles to destroy all the small laser towers on the floor, but not the ones on the walls. You must destroy each group of towers at once with one volley of missiles, and they must be destroyed all at once. The best way to do this is to lock-on to the furthest turret first, and so on up, then let your missiles fly. If done correctly, you should get some kind of confirmation from Wedge or Crix. Afterwards, the TIE Bomber will be unlocked.