Isochron Scepter + Pull from Eternity + Backdraft Hellkite + Karn's Temporal Sundering

Isochron Scepter

Pull from Eternity

Backdraft Hellkite

Karn's Temporal Sundering

How it works

1. Cast the extra turn spell and move to your extra turn.
2. Activate Isochron Sceptre to cast Pull from Eternity, putting the extra turn spell from exile to your graveyard.
3. Attack with Backdraft Hellkite to give your extra turn flashback, then cast it.
4. Repeat for infinite turns.