One of the cool things about Ni no Kuni 2: Revenant Kingdom is that almost every item that you would want can be crafted in your own Evermore Kingdom. While the low leveled weapons or items can be crated easily by having your citizens gather you materials, high-leveled items like armors, accessories and weapons need much rarer materials that require you to really farm. One of those rare materials are the Elemental Prisms that are hard to come by.
Here’s exactly where you can farm the Elemental Prisms:
The easiest way to farm for the Prisms is through the Swift Solution merchant that you find in various kingdoms like Goldpaw (after solving the mystery of the town) and Hydropolis. Once you have access to the Swift Solution merchant, you can do an errand for him to get rewarded with a Token of Gratitude tokens. These tokens can be exchanged for rare materials and one of them is a set of elemental prisms.
Fight Elemental Incarnate
As you fight enemies through the game, you will encounter an enemy called Incarnate that is based on its element. These enemies take the humanoid form that keeps running when you encounter it. Make sure to strike it down fast or it will flee.
Farm in your Kingdom
Find a citizen for your kingdom that is a Miner and you can farm a prism (if you are lucky). Note that you also need to raise the level of the mining shop to even get a wider selection of prisms that you can harvest.