You don't even have to be on Live just go into multiplayer and turn on classic mode.
Then go to the map crossfire.
When you spawn you should load up (this makes it more fun once you get to the end).
Pause the game and look at the map, as you can see it's just a big Z.
Go to the fartherest point at the bottom of the sideways Z.
You should see a building that you can not go behind. It's mostly blown to pieces lol.
Go up the stairs at the back and go up them. You should see a chair up against the wall on the right side of the 2nd floor.
Jump onto the back of the chair, then jump onto the bricks above the chair.
Then back up a bit, point your weapon up, then jump for the third floor (this could take a long time it took me 13 tries no lie).
Once you're up there get on top of the block of concrete leaning on the wall.
Then jump for the top floor (this should be easier than the first jump), then look out at the street.
Look to your left then you should see a green sign at the corner of a building, jump off the top floor toward it.
You will float in mid air, walk while your touching the sign until you can look down and see the barbed wire.y
You should have some grenades for suicide if you make a mistake. Walk across the gap to the other building, but make sure you stay above the barbed wire or you will fall and die.
Then you should see another gap, walk or sprint across it (I like to look at the building from behind it).
Then you are in a great sniping spot, but you can go farther... go to where you can see the roads behind the building where you see the roof of the building next to you.
Walk forward and it won't let you fall off. Be in the corner where you can't fall off and where the other building is next to you. Jump but not too far.
You should be kind of on top of the roof, jump again onto the roof, go to the other side where you see the water tower and the antenna's.
Sprint and jump to them.
Once again you'll float in mid air, go to the edge and look down onto the fence line where you can see the sniper rifle.
Jump down onto the fence line, you'll float ontop at about the level of the power lines.
Walk above the fence and go until you get to the turn, go around the turn until you get where you see a white gate.
Go above it untill you're level with a building, jump up to it and it has one more level.
Make it to where the top level is next to you, jump to the side until you're ontop.
You can float ontop of most of the building, even the blown up parts.
[more cheats for Call of Duty 4 here]