Wouldn't help to have a more descriptive info box, huh?

You should get this on the first mission to get the music box in the library

You get this once you get the ability to attack the enemies with melee, ranged and magic. You don't have to do any order, just kill one enemy with each weapon.

I personally really like this achievement. Seeming that it encourages the player to experience all aspects of combat to find out what they like best