Yup, shaman is very useful in the epic stage. It's also useful in arena.
There is no hard fast rule about it. I will focus on leveling & only using friends when I don't need it for the epic boss.
Depends on how u want to play & what is your focus in the game.
This epic boss is harder, so I guess u might not have enough friends to farm for jewels.
If u don't intend to get this boss armor, then everything goes.