For this achievement you need to play the Attack & Defend game type my preferred map is Presidio. If you were to try this legit, let's be honest all the opposing team would no doubt be guarding the bomb so to defuse it would be impossible.
So that's why you would be best suited to joining or hosting a boosting session with at least 6 players present. This will result in the achievement unlocking in around 5-10mins (when it is your turn).
Simply have a player from the losing side plant the bomb, at either site A or B, make a decision at the start of the game where the bomb is to be planted and run there for dramatic effect! Defusing takes all of 10 seconds to do so as mentioned you will get this very quick.