For the Farming Method, you will need the Shovel. You have to do the Quest for the Spear. So to start the Quest for the Spear, go to the Forge and talk to Dilia.
Now you will need to Talk to Lo.
After talking to Lo, you’ll need to Find 5 underwater ingots. Come near the bank of the sea.
Now you will need to dive into the water and look for the 5 ingots. These ingots are mostly near the Island, so don’t go too far away from the island, It can take some time to find them.
After collecting All 5 Ingots, Head back to Lo.
After getting the Spear, you don’t have to Continue its quest line. Just go and Talk to Dilia again to get the Questline for the Shovel. Go to Encounter Solla (she might be standing in the balcony).
Now you have to Find 2 Parts of the Shovel, the Handle and the Spade. Go to the left corner of the upper courtyard.
Now Return to Solla.
Talk to Solla again and you’ll get “Lost Treasures Part 2” Quest. To Complete it, you need to Dig 1 Treasure Chest.
To get a Treasure Chest, find a X around the Around and Walk towards it to start digging them up. Each X gives 1 Treasure Chest.
Now Return to Solla to Complete the Quest.
After Talking to her, you’ll get “Lost Treasure Part 3” Quest. To Complete it, you need to dig 5 Treasure Chests. Now Return to Solla to Complete the Quest. After Talking to her, you’ll get “Lost Treasure Part 4” Quest. To Complete it, you need to dig 10 Treasure Chests. After Completing “Lost Treasure Part 4”. You will be able to Repeat this Quest as many times as you want. Simply Talk to Solla again.
Through this method, you will keep getting 1000 Coins every time you dig 10 Treasure Chests. This will help you get a lot of coins easily.