Grub, Storied Matriarch // Grub, Notorious Auntie | Combat Celebrant

Cards

All permanents on the battlefield (
Grub as
Grub, Notorious Auntie,
Combat Celebrant hasn't been exerted this turn, and
Frostcliff Siege with "Temur" chosen).

Declare
Grub and
Combat Celebrant as attackers, choosing to exert
Combat Celebrant as it attacks.
Grub and
Combat Celebrant trigger.
Resolve the
Grub trigger, putting a -1/-1 counter on
Combat Celebrant and then creating a tapped and attacking token copy of it.
Combat Celebrant dies as a state-based action due to having 0 toughness.
Resolve the
Combat Celebrant trigger, causing you to untap all other creatures you control and causing you to get an additional combat phase after this one.
Repeat each combat phase, using the
Combat Celebrant token instead.

This combo has been estimated to be Exhibition (Bracket 1+).