Successfully complete Campaign 1 to unlock the Sniper Rifle Range mini-game.
Successfully complete Campaign 2 to unlock the EMP Cannon Range mini-game.
Successfully complete Campaign 3 to unlock the Cow Bouncing mini-game.
Successfully complete Campaign 4 to unlock the Assassination mini-game.
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.
Stay below a hilltop, hiding from enemies, rise up, fire a few rounds, then dip back down to avoid return fire.
You can also use the thermal scope during the day to clearly distinguish vehicles and soldiers.