The Fractal 40 farm is pretty self-explanatory. You do Fractal level 40 – The Molten Duo – over and over again, as quickly as possible, to earn Fractal Encryptions and other loot. The key to making this lucrative is speed and efficiency. If you’re having to wait on people during or between runs, or if your DPS is low, then your gold per hour is obviously going to tank.
Many people recommend bringing a party of mostly elementalists with 1 chronomancer and 1 PS Warrior, but the DPS can be swapped around if you have a capable and flexible group.
Fractal Encryptions are not always worth opening – sometimes it is best to sell them directly on the Trading Post. You can use this tool from loltools.net to determine whether or not you should sell them or open. Under “Fractal Encryptions” it will say something like “Use free keys then sell” or “Use Deeply Discounted keys then sell” to tell you whether the cost to buy keys outweighs the average gain from opening one of the Encryptions.
Here is a video guide on Fractal 40 farming, if you’d like an in-depth look: