To get Shaman, you need to lead the 2nd-placed Clan in the faction.
If you’re not a Clan leader, then you can make your own Clan by going to the ‘Clan’ page in the faction menu, and clicking to create a Clan. It requires a small amount of gold. After you create the Clan, you (and other Clan members) can increase its contributions using mystery crystals, bounty quests, and events like Guard the Goddess and Holy Blessing. If your Clan has the second-highest contributions at the beginning of a day, then you will be Shaman for that day. If your faction’s General is a Clan leader, then they can’t become Shaman or Elder, so you can ignore their Clan in this context. If the General’s Clan is first or second in contributions, then if your Clan comes third then you will still be Shaman. So you just have to lead a Clan, and have that Clan be 2nd-highest in contributions out of all Clans that aren’t led by your faction’s General. When the day begins, you will be made Shaman based on this.
The Shaman and Elder (or Lieutenant and Priest) positions are based on your faction’s top Clan leaders, not on BR or battles.