Epic mounts and legendary dungeons are usually the way to go. However, I discovered a third, easy but luck-related way to unlock this achievement.
When you reach certain 'milestone' levels, you're awarded a gift parcel with some fine goodies inside. Occasionaly, these parcels contain purple items, and picking them up unlocks the achievement.
This is of course completely luck-related, and you might never get an epic item this way. Nevertheless, I got the achievement on both versions this way - on levels 30 and 55 respectively - so the chance isn't that slim.
Additionaly, if you have unlocked the achievement on one platform, make sure to open the parcel on the other platform, as the achievement doesn't carry over!