Make sure that you have purchased The City DLC(for the Elementalist character), or that it's on free trial rotation, and activate it for your game. Choose the Elementalist as your character, and start the game.
During the game you'll encounter enemies of the Elemental type. Enter into battle with one, and you'll simply get the trophy without having to do the actual combat part. The first time you do this, you'll unlock the achievement.
Shouldn't take too long, but it all depends on how quickly you meet an Elemental enemy.
(If you also have the Firelands DLC, it'll make things easier by including more Elemental enemies.)