Roar of the Kha in exile (Roar of the Kha exiled by Isochron Scepter).
Isochron Scepter and Karn on the battlefield.
You control creatures that can tap to produce at least ({3} magic symbol) .
({1} magic symbol) available.
Steps
Activate Karn by paying ({1} magic symbol) , causing Isochron Scepter to become an artifact creature until end of turn.
Activate creatures you control by tapping those, adding ({3} magic symbol) .
Activate Isochron Scepter by paying ({2} magic symbol) and tapping it, casting a copy of Roar of the Kha without paying its mana cost, untapping all creatures.