Holding priority, activate Rootha by paying ({2} magic symbol) and returning it to your hand, copying Rishkar's Expertise.
Resolve the copy of Rishkar's Expertise, causing you to draw cards equal to the greatest power among creatures you control and casting Rootha without paying its mana cost.
When Rootha enters the battlefield, Mana Echoes triggers, adding ({C} magic symbol) for each Orc and/or Shaman you control.
Repeat from step 2.
If you control two other Orcs and/or Shamans, this combo nets near-infinite colorless mana.