After you drop into the fight from the air vent, defeat the enemy that you drop onto straight away by using the ground move to finish him off.
Once he is out of the way use counter when the blue halo comes above an enemy’s head and hit them a couple of times. When you have used the counter and a couple of hits on an enemy move to the next one. Keep using the counter when the blue halo appears above an enemy’s head and hit them twice.
Keep using this counter hit combo to defeat all the enemies in this area.