Stay hidden behind covers as much as possible, especial once you've take down your mark and enemy is in the area.

With that said don't get all guns blazing you won't get far.

Headshots are essential to the game and practice makes perfect.

For eve more stealth in your game knives are as good as any sniper rifle.

Use Assault Rifle only when your cover is blown. Keep both of your rifles upgraded to the max.

Upgrade your base for more storage and production.