Breath of Fury + Leonin Warleader + The Fire Crystal
Breath of Fury
Leonin Warleader
The Fire Crystal
Infinite creature ETBInfinite creature LTBInfinite combat damageInfinite combat phasesInfinite creature tokens with hasteInfinite death triggersInfinite mana creatures you control can produceInfinite creature sacrifice triggersInfinite untap of creatures you control
How it works
Declare the creature with Breath of Fury attached and Leonin Warleader as attackers.
Leonin Warleader triggers, creating a creature token.
Deal combat damage using the creature that has Breath of Fury attached.
Breath of Fury triggers, causing you to sacrifice the creature it is attached to, then attach Breath of Fury to the creature token, then causing you to untap all creatures you control and to get an additional combat phase after this one.