Benefactor's Draught in exile (exiled by Isochron Scepter).
Isochron Scepter and Karn on the battlefield.
You control creatures that can tap to produce at least ({3} magic symbol) .
Steps
Activate Karn's first loyalty ability by putting a loyalty counter on it, causing Isochron Scepter to become an artifact creature until end of turn.
Activate creatures you control by tapping those, adding at least ({3} magic symbol) .
Activate Isochron Scepter by paying ({2} magic symbol) and tapping it, casting a copy of Benefactor's Draught without paying its mana cost, untapping all creatures and drawing a card.