Start the mission "Rock And A Hard Place" on easy and playtrough the missio to the point where you see the Jet for the first time (it is a checkpoint). Before you see the Jet in the mission it is ALLOWED to die, but AFTER you have seen the Jet DO NOT DIE, or you have to restart the missio. Take your time and get from cover to cover without rushing, get the stinger, target the jet ant shoot. If you see a litle smoke track after the Jet then it is the Rocket, now wait till the Rocket hits the Jet and you will be fine.
Do not die after you have seen the Jet (checkpoint) or you have to restart the mission