You will be taught about parrying attacks quite early in the game. To do so, hold
to lock onto an enemy. Now press
the moment an attack is about to land.
This is easiest done against skeleton archers but it is possible to parry melee attacks (later on some enemies will have shields that can only be removed with a parry).
As long as you remember to parry every now and then, this will come naturally. If you want to boost this quickly, play until you reach the "Dirt Path" which is a small area on the way to Zameris*. There are two archers in this area - both on platforms off the main path. Use parry to kill both and exit the area. Return and they will have respawned. Rinse and repeat until you unlock the achievement.
*I've been using this video guide series
and it's been pretty good. Not mine - credit to Youtuber "WishingTikal"