Sniper Rifle Range mini-game:

Finish Campaign 1.

EMP Cannon Range mini-game:

Finish Campaign 2.

Cow Bouncing mini-game:

Finish Campaign 3.

Assassination mini-game:

Finish 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.

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.

How to save missiles:

By shooting 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.