Unlock Mini-Games
Complete the following goals to unlock the corresponding mini-game:

Sniper Rifle Range: Beat Campaign 1
EMP Cannon Range: Beat Campaign 2
Cow Bouncing: Beat Campaign 3
Assassination: Beat Campaign 4

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.

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.

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

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