This achievement requires you to disarm one of the High-Explosive Charges set by an enemy Soldier. To do this you must be an Engineer.
An easy level to gain this one on is the first mission for the RESISTANCE forces, Day 1: Getting Answers. The only main objective on this level is a set of two doors that your enemy will be attempting to breach through. This ensures that 90% of the time, 90% of the enemies will be attempting to plant a bomb.
Unfortunately, 90% of your team will also be helping you defend the doors, so I suggest you hold fire on any enemies until you have confirmed they are not soldiers. (A soldier is represented by a small shield with three rounds (bullets) in it, just to the left of his health bar.)
Once an enemy soldier has planted a charge, immedietely and single-mindedly bulldoze towards the door and hold X when in range to begin the defusing process. As I stated above, most of your team will be defending this room, they can deal with the intruders, you just want to be defusing.
Obviously, if online players are being non-compliant, you can set your Campaign settings to Solo play, whatever difficulty you choose, to make things run a little smoother.