You can save to protect your progress in this achievement. So after you’ve defeated a few enemies without losing any vitality, save your game. Then if you get hit, you can simply reload. You do not have to earn this achievement during a single encounter.
The easiest place to get this is during the prologue, before and after you fire the Ballista. In these encounters, small, manageable waves of enemies will attack. Remember, this doesn’t have to be done in one encounter, so the Ballista firing section will not impede your progress. Make sure to make liberal use of your block.