This can be done offline against bots and can be very simple to do
First: pick Classic Casual, Don't want any stray fire causing potential harm to your achievement
Second: Pick the map 'Dust' and Play as Counter Terrorists, not only do you spawn in a better place [In my opinion] This gives you easy flanks
Third: Save up some money for the AWP and when Possible buy it, the gun is a one hit kill in most stances
Fourth; While saving up money, watch your team mates and enemy patterns, they will practically be the same round after round, so if your team mates head right, go to the left door on the central corridor, the enemy will MOST LIKELY [Not always] be there to fire at your team of AI's, This is the better one of the two patterns but either one works
The second pattern is they all rush under to the opponent side, though you have the high ground and see them coming, there's risk you'll get hit so be careful
Lastly: line up your sights and pick them off one by one, if one of your team mates or you yourself get hurt, try again next round, you'll get it eventually
EDIT: According to Rasta Erva the achievement is glitched, can we get more people to confirm this please, thanks
Negative voters please state why and I hope this helps