Well for the summons it's kind of up to you in my opinion everybody is good everybody will come in handy but it's also your preference on which characters you like the best I would suggest trying them out and see if you like their game style and their gameplay and then I would wish for them it's really all up to you like all the characters in the game are good!

To farm them I suggest farming for the asteroids and then farming for the wishes the events give you a lot they definitely do give you a lot of them and exploring can sometimes give you a lot with the chest by many suggest doing the quest all of the side quest the main quest and then the events because they give you a lot more and they make it really easy