Shoot red barrels to trigger an explosion that will kill any nearby targets.
Green target missions are missions that progress towards completing a chapter. Side missions are where secondary weapons are used and can be repeated as many times as desired. If you have the required secondary weapon, farm those missions for money.
Always try to get headshots for a money bonus. However, you can also attempt to get perfect shots. To do this, enter focus to slow down and time and keep the reticule inside the red boundaries. After a moment, a small white crosshair will appear in the center of your target's head. Shoot exactly on that mark to earn a perfect shot and get a gold reward. Gold is the game's premium currency.
Tag enemies before taking your first shot. Focus on your objectives first, followed by targets that are marked as a threat. Threats are usually enemy snipers. If not killed in time, they will focus on your position and kill you. Take them out with suppression shots. They are usually found alone, on top of roofs.