Shoot through floor
If you have a guard in a tower he easiest way to kill him is to shoot through the floor. This works on barracks as wll.
Enemies don't shoot
Go project IGI installed folder then goto commen\ai backup the folder ai, There will some files like "civlian.qvm" ,"guard.qvm" like that names of enemy's copy the file "civlian.qvm"[ctrl + c] and paste it [ctrl + v] in that same folder, if willbe like copy of civlian.qvm then rename enemy's files into some other names then then rename copy of civlian.qvm into name of enemy's files like guard etc.. Then, start the game and enemies won't be shooting at you (they will run like civlians).
Third level tip
When you are in third level, first try to shoot guards from a distance where they can't see you. from outside i.e from mountains hit soldiers ones or twice so that they understands that you are their & they come outside, run on mountains or the position where game starts, then they watch over mountains & turn back to go. Then run & kill them from back (it works,which is quit easier than natural).