Roar of the Kha in exile (Roar of the Kha exiled by Isochron Scepter).
Isochron Scepter and The Blackstaff of Waterdeep on the battlefield.
Creatures you control can tap to produce at least ({3} magic symbol) .
({U} magic symbol) available.
Steps
Activate all mana-producing creatures you control by tapping them, adding at least ({3} magic symbol) .
Activate The Blackstaff of Waterdeep by paying ({1} magic symbol) ({U} magic symbol) and tapping it, causing Isochron Scepter to become a 4/4 artifact creature.
Activate Isochron Scepter by paying ({2} magic symbol) and tapping it, casting a copy of Roar of the Kha without paying its mana cost.
Resolve Roar of the Kha, untapping all creatures you control.
Activate all mana-producing creatures you control by tapping them, adding at least ({3} magic symbol) .