Declare Grub and the creature with Breath of Fury attached as attackers. Grub triggers, putting a -1/-1 counter on the creature with haste, and then creating a tapped and attacking copy of that creature.
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 token with haste, then causing you to untap all creatures you control and to get an additional combat phase after this one. 
Move to your next combat phase, and declare Grub and the token that has Breath of Fury attached as attackers.
Grub triggers, putting a -1/-1 counter on the creature with Breath of Fury attached, and then creating a tapped and attacking copy of that creature.
Repeat from step 2.

Infinite ETB

Infinite LTB

Infinite combat damage

Infinite combat phases

Infinite death triggers

Infinite mana creatures you control can produce

Infinite sacrifice triggers

Infinite untap of creatures you control

Grub, Storied Matriarch // Grub, Notorious Auntie

Breath of Fury

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.

Infinite creature tokens with haste

Combat Celebrant

The Fire Crystal

Cast Glasspool Mimic by paying {2}{U}. Choose to have it enter as a copy of Grub, Storied Matriarch.
Put Grub, Storied Matriach into your graveyard due to the legend rule.
When Glasspool Mimic enters, it triggers, returning Grub, Storied Matriarch from your graveyard to your hand.
Activate Agatha's Soul Cauldron by tapping it and exiling Civilized Scholar from your graveyard, putting a +1/+1 counter on Glasspool Mimic.
Activate Glasspool Mimic using Civilized Scholar's ability by tapping it, causing you to draw a card and discard Grub, Storied Matriarch. Glasspool Mimic will then untap itself and transform, but it will transform again into Grub, Storied Matriarch.
Glasspool Mimic triggers, returning Grub, Storied Matriarch from your graveyard to your hand.
Repeat from step 5.

Infinite card draw

Infinite draw triggers

Infinite self-discard triggers

Glasspool Mimic // Glasspool Shore

Agatha's Soul Cauldron

Civilized Scholar // Homicidal Brute

Cast Mirrorhall Mimic by paying {3}{U}. Choose to have it enter as a copy of Grub, Storied Matriarch.
Put Grub, Storied Matriach into your graveyard due to the legend rule.
When Mirrorhall Mimic enters, it triggers, returning Grub, Storied Matriarch from your graveyard to your hand.
Activate Agatha's Soul Cauldron by tapping it and exiling Civilized Scholar from your graveyard, putting a +1/+1 counter on Mirrorhall Mimic.
Activate Mirrorhall Mimic using Civilized Scholar's ability by tapping it, causing you to draw a card and discard Grub, Storied Matriarch. Mirrorhall Mimic will then untap itself and transform, but it will transform again into Grub, Storied Matriarch.
Mirrorhall Mimic triggers, returning Grub, Storied Matriarch from your graveyard to your hand.
Repeat from step 5.

Mirrorhall Mimic // Ghastly Mimicry

Combos with "Grub, Storied Matriarch // Grub, Notorious Auntie"