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

Hint: Surprise kill:
Stay below a hilltop, hiding from enemies, rise up, fire a few rounds, then dip back down to avoid return fire.

Hint: See enemies easily:
You can also use the thermal scope during the day to clearly distinguish vehicles and soldiers.

Hint: Saving missiles:
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.