This does not have to be achieved in one round.
Enter Terrorist Hunt on Single Player and choose Kill House as the map. Set it to Casual difficulty so that it doesn't become an annoyance and set density to High. Equip yourself with a decent pistol because that's all you're allowed to use on the ropes!
When you start, you'll be facing a staircase. Head up the staircase and onto the 'catwalk', which is the long platform down the side of the map. There are two rappel hooks for you to use, you should get on the first one.
Fire a bullet to attract attention to yourself and they'll come running to try and take you down. Take them down one by one and you will unlock the achievement after 25 kills on the rope.