Confirmed. After charming 3 coldain or kromrif "soldier" mobs we got this achievement. Once the soldier mobs were charmed they said, "A coldain soldier shouts, 'I have not forgotten who my real enemy is, A Kromrif guard!'" and then they ran off.
Note: The achievement wasn't auto granted once we had charmed 3 soldiers but after killing Zeixshi-Kar.