Some machine gun levels feature yellow barrels, usually with enemies nearby. Shoot those barrels, to detonate them and kill nearby enemies. This will result in bonus cash.

After dropping into a zone, use the time where the enemies are not yet alerted to scout the area. The enemy will not be alerted until you fire.

Move your crosshairs over an enemy until it is tagged. Tagged enemies will have a red diamond appear above them, allowing them to be found easier. Tag all enemies before firing your first round.

Headshots will result in bonus money. This will also bypass their body armor when it eventually is used.

Upgrade your sniper rifle as often as possible, followed by your secondary weapons. The final sniper rifle upgrade is the silencer. It allows you to fire without alerting the entire area.