The fastest way I found to grind this was on the level “Close Call”.
You can select this mission from the chapter select on the main menu.
Shortly after a few rooftop gunfights you’ll drop into a small courtyard area that will have a few regular enemies and a shielded enemy.
Shoot your regular enemies but keep the shielded enemy alive as you’ll be using them to grind counters.
They’ll try to attack you with their baton and you can counter that with RB.
They’ll keep trying this attack and you can keep countering it.
They will not die from your counters, keep going until you get the required amount needed for the achievement.