This achievement requires you to buy the Reflexes skill along with the Superior Deflection upgrade. The upgrade that gives you extra time may also be useful, but is not necessary. If you don't want to buy these upgrades for your primary play, save, get the achievement, then reload.
Once you've bought the upgrades, find one of the officers wearing the red coats. Try to kill any other guards in the area so they don't interfere, then get the attention of the officer. They're pretty much guaranteed to fire a gun at you if they spot you from a distance. As they fire you need to hit block to deflect the bullet back at them. Note, that a single deflection won't kill them. It probably takes about three successful deflections before they've taken enough damage.
One complication is that after the first shot, they'll likely close to melee range with you. Try not to swing your sword at them, just block and when you get a chance grab them into a hold then throw them away from you. At this point, they'll temporarily lose sight of you and when they see you again (keep a little bit of distance), they should reset to firing a gun at you again. Keep repeating until you get a deflection to kill them.