This write-up contains spoilers.
Don't say you weren't warned.
The hardest level in Hitman: Codename 47 is "Say Hello to my Little Friend" in which you kill a Scarface wannabe and blow up his drug lab (with a bomb he has conveniently purchased for you and set on his desk). Some would say that the hardest level in the game is "Plutonium Runs Loose" but technically it isn't really any harder than "Say Hello..." -- it's just infinitely more boring. I'd rather chew my own arm off than finish "Plutonium...", but I've come close to going back through "Say Hello..." just to show off.
To complete "Say Hello to my Little Friend" the Viet Cong way:
Bring the sniper rifle from the previous level and go into sneak mode behind the box at the beginning of the mission to snipe Pablo (you can see him through a window in the mansion) and the three tower guards that are lined up in front of you. Drop the sniper rifle and exit sneak mode before the patrol comes along and spots you with it (sniper rifles are not standard goon issue).
Walk to the right along the fence to reach the compound's entrance. Make a mental note of the number and position of all the stationary guards outside of the fence along your route. Enter the compound and climb up the first tower to your left as you go through the gate. This is the tower without the sniper rifle but with a whole shed load of sniper rifle ammo. You won't be able to pick up all of it before the guard in the tower flips out and goes postal on you however you should be able to get most of it if you're fast and careful. The guard will talk to you four times before he starts shooting.
Go back to where you dropped your rifle and grab the vest and a knife from the box then go into sneak mode (very important) and pick up the rifle. Most likely at least one of the tower guards you sniped earlier fell out of his perch and has attracted the attention of some patrolling guards: Snipe them. If a patrol comes along outside the fence snipe them as well. Staying in sneak mode, move along the fence the way you just came (this time hugging the cliff face). Snipe all the guards that notice the dead guards you've already shot and any tower guards you can draw a bead on. Anybody you see outside the compound needs to die before they see you so the foreground should be your top priority. Don't waste ammo on guys inside the compound who are just guarding things (doors, gates, etc.) -- only shoot ones that have noticed the dead bodies.
Using this "honey pot" method I was able to kill all the patrols, the guards standing outside the fence and all but a few of the farthest tower guards without any of the remaining guards being alerted. If you can do this then it's a fairly simple matter to kill all the guards in the mansion silently (the knife is better than the piano wire). Exceptions to this: The two pesky guards in the fountain room who have to go noisy and two really peripheral guards in one of the outer entry rooms who can be left alone. This leaves only the guards at the turnstile checkpoints, in the lab and at the plane to actually shoot at you when the excrement hits the rotating blades.
Speaking of the lab guards, once you're finished inside the mansion go into the lab through the stairwell entrance. There is a tiny "mud room" on this side of the bunker which is useful because if you hang around in here for a bit (and get a little lucky) all the guards in the lab will line up outside the inner door and you can mow them down when you come in without a lot of hassle. How considerate of them.
Some of this I learned from a FAQ, but much of it including the twist of killing nearly everybody before anybody even knows you're there is my own -- the FAQ said nothing about getting the additional sniper ammo, instead suggesting that you exhaust your ammo then go in with guns blazing (something which I tried numerous times and simply couldn't get to work).
As a postscript, here's a miscellaneous tip I discovered myself in a later level: When your sniper rifle is in its case metal detectors will find it but people looking at you will not. Not only this, but people standing around outside of hotels won't think anything of a bald guy with a bar code tattooed on the back of his head dropping a briefcase before going through a metal detector then picking it up again through the bars from the other side. What's really funny is how these same people have a strict shoot first and ask questions later policy towards an otherwise unarmed man setting a metal detector off with a knife in his pocket ("But it's my favorite steak knife! Ow!")