0. Patch the game before you play. Version 1.2 seems more stable and is easier.
0.5 Don't sacrifice playable frame rates for visual effects. Jerky framerates will seriously hamper your enjoyment of the game. Go for a setting that gives you around 30FPS and stick with it.
1. In stealth mode when you move you use energy very quickly and when you run out of energy you de-stealth. Crawl or crouch to move more slowly and reduce the rate of energy consumption.
2. When you run out of energy in stealth mode you switch to armour mode BUT you still have no armour because you are our of energy.
3. When you shoot in stealth mode you deplete all of your energy, de-stealth and switch to armour mode (with no amour).
4. 2 and 3 above explain why it is not a good idea to use stealth mode to run into a group of enemies and then try to quickly shoot them all.
5. Silencers are great for stealthy play but they reduce the damage a lot so you absolutely need to aim for head shots.
6. Switch the automatic rifles (SCAR / FY) to single shot mode ("X"). You do more damage and have less recoil. You can still fire quickly by tapping the mouse button.
7. Strength mode steadies your aim. Essential for long range sniping.
8. Speed mode can sometimes allow you to run out of harms way.
9 The SCAR and FY make pretty decent sniper rifles in single shot mode with a sniper scope. They do less damage than the precision rifle but head shots will still kill. Added advantage that you can use a silencer.
10. For some reason the Koreans can't see your laser aiming beam when you are stealthed. Silenced pistols / assault rifle with laser pointer allows for multiple head shot kills from a stealthy hiding place.
11. The twin pistols are actually a very useful weapon. As good as the assault rifle at short to medium range. Fit silencer and laser aiming.
12. Ways of taking out Koreans in Nano-Suits: At short range two blasts of the shotgun. At medium to long range a burst of incendiary ammo from the FY to the chest.
13. The Gauss rifle is the best weapon for taking out all kinds of aliens. It it will see you through to the end of the game. In the underground levels when you haven't got the Gauss rifle yet use a shotgun.
13. The rocket launcher has a homing mode when you press right click. Handy for taking out helicopters.
14. A helicopter on your tail can be difficult to shake. If you have a rocket launcher 2 to 3 rockets should kill it. If you haven't then use stealth mode to run to get to a group of trees and hide. Do not stealth and stand still because you will get bombarded with rockets.
14. Pick up the precision rifle and the sub machine gun to get the sniper scope and laser aiming respectively but don't bother bringing them with you. The assault rifles (SCAR / FY) can do both jobs just fine.
15. Cough..cough...F5 is the quick save button.