In order to earn this achievement, you must attack enemies that are equipped with a shield at least one-hundred times.
You can earn this in your playthrough, but if you have the extra time on your hands, just find a group of shielded enemies, preferably the weaker ones. Just stand in front of them and hit their shields. You'll know that you'll do it right when your assault is reflected by their shield. If you hit them, it will not count. So just hit their shields one-hundred times and the achievement is yours. Have fun and good luck!