Once you unlock Intervention Battles, located in The Girl's Tearoom, there will be a number of Intervention Quests available. They require 2 Arma Gems (obtained by completing boss battles and through some of the Colosseum battles) each to do.
The specific one you're looking for is called "A Duo of Three" which you will have to complete to get this achievement.
It's fairly easy, though I suggest be in the mid 20s level wise. I was level 33 and had zero problem. Insta kills on all of them.