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The grind for this achievement can be rough, but inside the guide will reduce the time by a considerable amount.
How to Get
As mentioned in previous guides, the fastest way to grind out the 50,000 gil is to go to the Diablo stage, Ruins of Sarudnahk. What none other mentions is that its possible to do it quicker in this area. First off, make sure you have gotten every piece of slot gear for this stage. The less items the drop, the more chances that gil will drop.
I suggest to do this at the same time you are attempting to gather the 2nd piece of the amulet, so that you can chose to play the map as Kaeris. Her combo attack is a big AoE which speeds up the grind even more. Run the stage until you beat the boss, collect the amulet, and open the town portal. Do not go through, instead return to the 2nd save point, save, exit to main menu, and continue. Everything except for the boss will respawn. Follow the route shown below to kill each pack of enemies until you reach the 1st save point on the map, and repeat.
It used to be you could gain between 5-8k in about 7 minutes for clearing the map, and running out. This way nets the same amount in half the time. You can get the 50,000 easily in about 15-20 minutes.