Hello guys! Well, as you all can see by the title, I'd want to know any sustainable ways to earn zeny in the game. Many of you probably would suggest that I should sell cash items or, mainly, do instances and sell the items gathered. That's the point: I have no conditions to buy cash items and sell them and I have many difficulties to do some instances, like Sky Fortress and NTF. The instances that I can do are OGH, Charleston, Endless Tower (but not all the floors) and Central Laboratory. But there is another problem about those instances: I have to spend a lot of money buying potions and other items in order to survive the forementioned instances, to the point it makes the things really unsustainable, since I spend a lot of money with things that depends very much on the luck to drop valuable items.
So, if you have any tip for me that will help me, it is very welcomed!