Guide: During the Second AC-130 sequence, Just after the squad get into the convoys, keep providing cover for them using the machine gun and 40mm rounds. if you're zoomed in you will see one convoy chasing the squads past some broken building's (Unless you destroyed it) just after that they turn left and drive forward (So 12 o'clock to you). look down and there will be 3 tanks, close together, switch to the 105mm and fire at them. Aim for the middle one and you should take all 3 out, if not, go back the the last checkpoint and try again.
Good luck and have fun