When you get to Serpent's Coil, the enemy there will always be Jells and always appear in numbers of three or more. They are immune to physical attacks. When you are at level 10 or greater, use only elemental Arcanas. After the battle, notice that you will get Arcana Gems exactly according to the Arcana used to kill the Jells. Every battle can result in three or more Arcana Gems. Each gem can be sold it for 100 Gellas. Every enemy you kill using an Arcana is bound to give away Arcana gems.