yea the auto aim can suck , try a different weapon, maybe level up the gun a little bit, with the Uzi at maxium level you can shoot 2 at once. keep trying, you will get it. BTW how much percentage of the game have you been through?
also if you push I think it is R2 and L2 you can scroll though the list of lock on targets if he is aiming at a tire just push it real fast and he will keep locking on to different things, I think you can just do that until you get to the target you need to hit.