For this achievement you will need to get Shield of the Underworld (600 gold Active item). When the enemy is low and still attacking you pop this active and a percentage of the damage done to you will be reflected (also blocks all life steal so it is great against hunters with 1+ life steal items).
Another way to get this is to use Nemesis (Assassin). Her 3rd ability activates a shield around her that will also reflect a percentage of the damage done to her while the shield is up.