circle strafe is an essential trick, set the arrow keys to walk you sideways and then practise walking sideways and using the mouse to circle some low level opponents while you shoot em, I found killing em at long range then backing off to stop hosts of them chaseing you works well, they are pretty dumb, so just stalk em and keep nibbling away at the edges, the main bugbear are the rocket launcher guys, you often need to die several times to find out where they are firing from since the rockets apparently travel faster than sound, also, getting a tree or rock between opponents and you makes them lose contact, and they home in on sound, so when you shoot someone, relocate ASAP, you can draw em to one point with some fire and be up the slope overlooking them by the time they get there, sniper ammo is most useful, clear the area then use "v" to lie down and snipe out the bad guys on max zoom, later levels get insane though, VERY hard and IMO silly, the last few levels are harder than the whole game, I`m still stuck on fighting my way through trijan central with no ammo after being thrown from a helicopter, as if I would have survived a one thousand foot fall with no chute anyway.
Mentor.
PC1: P4 hyperthreading 3ghz, 1gig mem, 2x160gig hd`s, Nvidia FX5900 gfx, 6 way surround sound, PC2: AMD 1.2ghz, 512mb ram, FX5200 ultra gfx, stereo 16 bit soundblaster, ups.
