DervishG 15

"Inner Focus" is the Northern Technique attack (LT + RB) of the Dual Blades weapon, this has to be performed when having a bar of energy filled (the orange bar just above the blade). To perform a weapon technique, you need to have put a skill point into "Weapon Techniques" in the skill tree.

You can simply spam the weapon techniques inside or outside of combat to get this achievement to unlock, as long as you have the energy bar for it. Once you perform 100 instances of "Inner Focus" the achievement will be yours.