Shoot the red barrels to blow them up and injure anything nearby.
Headshots
Headshots are always a critical hit, and usually will kill your target. Always try for headshots in long range missions where you have a time limit.
Secondary items
After you complete a mission, you will sometimes be prompted to watch an advertisement video after returning to the mission selection screen. If you watch it, you will get a secondary item as a reward.
Spec ops missions
Complete Spec op missions to earn plasma cells, which are required to unlock the next tier of upgrades for your weapons.