Long Distance Shot: Zoom in on enemies that are in the distance using the sniper scope and then fire at them without a lock using the canon or unguided rockets.
Saving Guided Missles: Try not to waste your guided missles as you'll need them to take down enemy helicopters. A good way of getting extra damage without using the guided missles is to use your canon and fire at fuel barrels and other explosive objects that are next to targets.
Guerrilla Warfare: Use guerrilla tactics like staying hidden below a hilltop and suddenly rising up to fire off a few rounds with your canon before descending back to your hiding place to avoid return fire.
Identigy Enemies More Easily: Use the Thermal Scope during the day to clearly identigy enemy soldiers and vehicles.
Unlock Sniper Rifle Range Mini-Game: Complete Campaign 1.
Unlock EMP Cannon Range Mini-Game: Complete Campaign 2.