Go on Terrorist Hunt and select the 'Kill house' map. Then put the difficulty to Realistic (if you feel you aren't very good on the game then you might not be able to do the realistic difficulty. It also works with Normal difficulty!). Then put the enemy density to either low, medium or high (depending on whether you want a challenge or not, if so choose the high enemy density option). Then start the game.

Once it has started run to the top of the stairs on the right hand side and take cover on the wall. Move your men to next to you (facing the stairs to the next level) and make them hold. When you have done all this then if you look around the corner you will see a rappelling rope. There should be a lot of men that come down the rope (depending on the amount of enemy density). Kill the enemies when they are on the rope and you should get 3 marksmen points per kill! Do this for 2 hours and you will have the maximum level for marksmen (hopefully).