To be able to unlock this achievement you have to upgrade the Arrow Redirection ability in the Training tree to level 2. This will allow you to hold RT in order to deflect arrows back at the shooter.
Once this is done you have to find an enemy armed with a bow or a crossbow. Many enemies carry them, but Bandits are often equipped with them so explore the forests around Vergen.
Bear in mind that this achievement may not unlock first time. If it doesn’t, swap swords and try again. This should do the trick.