Use the sniper scope to zoom in on an enemy, then you can shoot them with unguided rockets or the cannon without a lock-on.
Mission 3
First thing go to stealth mode(square)and tag the tower patrols quickly then disable the jeeps. Do the same for the next set of towers. When the tents come up tag the guy with the backpack and then disable the jeep then kill the other two. The next tent do the same. Then after the special Ops are out. Go to stealth mode fast it turns off after their out. Then turn on the thermal vision too, then kill the 4 patrols and follow the special ops until you see the smoke markers, then of course land and pick them up and your good to go! And that's from an ace!
Saving missiles
Shoot fuel barrels when they are next to enemies using your cannon.
Thermal scope
Use the thermal scope during the day so you don't confuse vehicles with soldiers.