Go up to a tank and order one of your men on to the tank if it fails the first time order that solder to stand down and repeat when he is inside switch to him and select the knife from your inventory move to the centre of the torret and stab you will see a puff of blood and hear the enemy die now the tank can't fire at you. But remeber to kill the machine gun man first.
To order a men to move hold L1 then press circle and toggle the positioning icon were you want him to go and press circle again.
I have discovered this myself while playing Desert Storm (Multiplayer). You know how when you advance further through the level, when you rescue Emir, enemy soldiers suddenly sometimes appear behind you, well I have discovered that you can keep them under control while your partner clears the rest of the level, so he/she doesn't have to worry about enemies appearing behind them. Have Foley lay down in a good sniping position facing where you fist started, (make sure Emir is laying down also, because emir goes where Foley goes), and when an enemy comes, (1 at a time), he should stop at a point and usually kneels down. When he kneels down, he won't shoot right away, so that is the perfect time to get a head shot with the sniper rifle. If the enemy does start shooting,, quickly take him out, even if it means not getting a head shot. Keep killing the enemy soldiers until your partner is ready for you to meet up with him/her at the extraction point. When your about half way to the extraction point, you might want to stop, get into a kneeling or laying sniping position, facing the way you just came from, and take out the guy that was following you (May have to wait a few seconds). Taking out that guy will basically be the last guy you have to worry about, so after taking him out, continue to head to the extraction point.