one way for the south pass , is to toy with the machine gunner ,never going pass the tree ,at some point they will come after you ,so if you time it right, you shoot the 1 -2 guys by the tree[head shots], in front of the tunnel ,you can grab their guns and ammo using the tree for cover. then I just use the north pass ,running over the tracks on the east side between the bridge and rail car.could be a guy there- just jump over him to the other side and run to the left. it's been awhile but I think he's a skalker so don't kill anyone at the bridge.
-also if you take on the mil base with lots of saves you can get lots of sellable guns and ammo before they respawn ,just take the guns to the ne stash where the tree is over the fence ,thats where you would get into the base using the junk at the back to jump into the base ,kill a few- take their ammo save ,kill afew more ,save until ,they are all dead .loot the base stash the goods ,make many trips to the trader, after that you should be good to go ,money good guns-ammo. buy armor if you can , not sure if any stashs in the roof\attic of mil base-jump on the backhouse to get on the roof.
-just a note for the trader ,try to block the door open, drop a gun ,move a grate ,I am now playing the first one and that door wouldn't open 1\4 of the game,after smashing the edge of the door[after tip] it moved a bit , I did get it open ,and used a air tank to keep it open ,but that door did not open in cs also last year when I played it., so put something in there .well know bug. there is a patch\mod to open it.
here's a guide if you want , lots out there --http://www.supercheats.com/pc/walkthroughs/stalkerclearsky-walkthrough01.txt