As of the time posting this, which is still fairly early in the games life, this is the quickest way of leveling to 50 to my knowledge. I'll update this if this ever gets patched out. While I haven't gotten the achievement at the time of writing, I saw my friend try this method and it boosted him fairly rapidly.
During the mission "Guardian Hunter" where you have to capture a Guardian Fire Wraith, go to the area where you have to capture the Fire Wraith. Rather then following the objective and capturing the Fire Wraith when you find it however, kill the Fire Wraith and watch your level, cash flow, and moonstones soar as the Fire Wraith keeps spawning in the same spot and at a rapid rate to boot. If you have a Nisha, or a high level friend who is playing as her, using her Showdown skill will allow you to kill the Fire Wraiths even faster and keep it going more or less infinitely, so its a good place to get the Executioner achievement to boot. That bein said, its still a good place to boost up your level with any character really, long as you can kill the Fire Wraith's quickly.
Hope this helps, and leave me some constructive criticism if you find the solution lacking!