Spoiler inside, containing name of enemy you will fight to get achievement----this is for a mini-boss battle.
There are three round while fighting this foe. The first round you just need to counter attack and then cape stun, followed by cape stun.. (2 attacks of this)
The second round you need to double tap A and avoid the aerial attack followed by a cape stun, and then beat down. You will have two sections of this, the second section you have to avoid the aerial attacks twice.
The third round is countering three times, cape stun, and then beat down.
Not too difficult. This is story related and can't be missed.