Guards
Guards have three different alertness levels. Curious guards will try to check on something going on. Suspicious guards are those that find a dead body. Alarmed guards are created by shooting a target in front of them or when you hit them with a non-lethal body shot. When the radio icon appears above their heads, you only have a few seconds to eliminate them out before they call for help.
Kills
Always avoid killing targets in plain sight. Guards will alert other people and your target will move to safety. The only time it is usually safe to kill in the open is usually during the extraction process, when there will not be enough time for witnesses to sound the alarm. Look for ways to dispose of bodies. Consider using water and the sides of buildings when available.
Rifles
New rifles can be unlocked by naturally advancing through the game, reaching a certain position on the leaderboards, or by collecting the required number of parts. Specific weapon parts will corresponding to various targets.
Various General Tips
Save up on the Hold Breath feature by cancelling it as soon as you make the shot. If you do this every time, you will be able to use it throughout the level, without waiting for a refill.
You can earn more points by being creative, so observe the targets and regular people carefully before taking any shots.
Once you complete the primary objective, all optional ones disappear so try to do them first.
In case you mess up and kill the main target first, use the Hold Breath feature to beat the clock and execute the secondary target in those few precious seconds.
Use the purple marker to mark a person of interest and keep an eye out on his movement.
A double headshot or some other creative kill will get you a lot more points, so keep your eyes open.
There is a secret rubber duck on the roof of the main building that you can find to get more points. There are also 5 hidden laptops around the map, but one of them is only visible in the night.
The text that comes up during the loading screen is almost always full of helpful information.