You can actually get 2 lunges onto one person and it counts towards this achievement.
I did this in the 2nd area after you break free of all the tubes. They keep sending two men down..It's usually after the overhead radio but sometimes not. They make it obvious they are there though.
So you press RB to lock and then LB to lunge. Just hit each enemy once with a lunge...Run away and then relunge at them. This gives you 4 lunges since there are two in the room. You can only perform 2 lunges per enimey though.
You could also use this through out the whole game to get this achievement to make it pop quicker.