The achievement is unlocked on the 100th successful parry. You must have a shield equipped
. A parry is when your
block is timed to coincide with the enemy's attack. Enemies have animations preceding their attacks. So, it's a matter of learning their 'tells' and blocking at the same time as the attack. If your
timing is not precise, you merely block. When it is precise, you'll get audio and visual feedback signifying a successful parry. If you are a melee fighter, you will parry often and you will unlock the achievement through normal play.
If you are a ranged or mid-ranged fighter--staff, scepter, bow, chakrams, etc.--you are normally out of parrying range during normal gameplay. The Boggarts are good enemies to grind this achievement out. They have two attacks--a jumping attack and a side by side attack. Both attacks can be parried. The animation that precedes either type of attack is very distinctive and clearly telegraphs the strike. It is easiest against one opponent but you'll reach the 100 count faster with two or more. A successful parry knocks back all enemies in your vicinity so you do not assume unacceptable risk fighting more than one opponent.
The very short video below shows the technique and the achievement unlocking on the 100th parry. Note the sound and visual embellishment that tell you that you have successfully parried rather than just blocked.
Note: If you are using an Elite controller or other controller with a stop that shortens the
pull, this may impede your blocking and parrying abilities. Make sure the stop is not used and that the
has the full range of motion.