After you go threw stealth training and go back to the base it will be under attack. When you reach the front of the base enter stealth mode. Stun the tanks, half tracks, and jeeps, then shoot the men.

Use the sniper scope to zoom in on distant enemies, then shoot them with the unguided rockets or cannon even without a lock.

Shoot fuel barrels and other explosive objects that are next to targets with your cannon to get extra damage without using missiles. Save your guided missiles for other helicopters.

You can also use the thermal scope during the day to clearly distinguish vehicles and soldiers.
Stay below a hilltop, hiding from enemies, rise up, fire a few rounds, then dip back down to avoid return fire.