I achieved this on Lake without any cheats. Be skillful enough and it's easy. I'm totally not very good and I could do this.
First try and get a good lineup of accurate weapons: CT SMGs (p90, mp7, mp9), the two scoped rifles, the two long shotguns, the m249 machine gun, and one or two of the heavy pistols.
At the start head to the house upstairs bedroom level and you can safely snipe off easy bots out in the field (or other windows also if you can), granted you snipe smart (keep moving, be patient, don't repeek, know when you're burnt). This is easier to do on CT team; CT spawn is too close to the house and CT tends to raid the house more. For when the enemy starts raiding the house, set a "door trap": The door to that upstairs bedroom, open it so it swings out. Do this and a bot will have to close the door before they can open it. You'll hear the door close and you can either hang back until they go away or prepare to shoot as they come through and get another kill. Either way, set the door trap back before going back to sniping. You can also hide in the bathroom for as long as you want as long as you don't snipe from there. You can take a break, go to the bathroom IRL, or wait for the enemy team to graduate to shotguns or pistols to make them easier to shoot and knife.
When you get to shotguns yourself: if you don't want to wait for someone to get close enough to the house, go to the basement and try to lure someone through that door facing T-spawn (open the door, fire out, then close door and wait) Or do what I do, peek out and if it's safe, look for a enemy facing away that's close enough, then run out quickly and knife them in the back since a knife kill bumps you up a level automatically. You're going to have to do this to get that final knife kill. And pistols: be patient, take your time, wait for the best opportunity, and concentrate on getting that headshot.
There's other places to snipe: Back of the house underneath the diving board, I find Bombsite level floor makes it easy to get head shots, there's even a way to get to the roof of the house to snipe (Top upstairs level, back of the house side facing CT spawn, you can crawl out the window onto the roof) but the roof leaves you exposed, so the best place is that upstairs bedroom.