Cast Mighty Mutanimals by paying {2}{W}{W}.
When Mighty Mutanimals enters, its first ability triggers, creating a 2/2 Mutant creature token.
When the Mutant enters, Mighty Mutanimals' second ability triggers, putting a +1/+1 counter on Basking Broodscale.
Basking Broodscale triggers, creating a creature token.
When the creature token enters, Mighty Mutanimals' second ability triggers, putting a +1/+1 counter on Basking Broodscale.
Repeat from step 4.

Infinite +1/+1 counters on a creature

Infinite creature ETB

Infinite creature LTB

Infinite colorless mana

Infinite creature tokens

Infinite death triggers

Infinite creature sacrifice triggers

Mighty Mutanimals

Basking Broodscale

Activate Viscera Seer by sacrificing another nontoken creature that doesn't have a -1/-1 counter on it.
When the creature dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When the creature enters, Mighty Mutanimals triggers, putting a +1/+1 counter on the creature.
Resolve the Viscera Seer ability.
Repeat.

Infinite scry 1

Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight

Viscera Seer

Activate Ashnod's Altar by sacrificing Mighty Mutanimals, adding {C}{C}.
When Mighty Mutanimals dies, Nim Deathmantle triggers, causing you to pay {4} to return Mighty Mutanimals from your graveyard to the battlefield and attach Nim Deathmantle to it.
When Mighty Mutanimals enters, it triggers, creating a creature token.
Activate Ashnod's Altar by sacrificing a creature token, adding {C}{C}.
Repeat.

Infinite +1/+1 counters on creatures you control

Nim Deathmantle

Ashnod's Altar

Activate Berta by paying {1} and tapping it, creating a 0/0 Fractal creature token, and then putting a +1/+1 counter on the Fractal.
When the Fractal enters, Mighty Mutanimals and Intruder Alarm trigger.
Resolve the Intruder Alarm trigger, untapping all creatures.
Resolve the Mighty Mutanimals trigger, putting a +1/+1 counter on Berta.
Berta triggers, adding {1}.
Repeat.

Infinite mana creatures you control can produce

Infinite untap of creatures

Berta, Wise Extrapolator

Intruder Alarm

Activate Barrenton Medic's first ability by tapping it.
Activate Barrenton Medic's second ability by putting a -1/-1 counter on it.
Flourishing Defenses triggers, creating a creature token.
When the creature token enters, Mighty Mutanimals triggers, putting a +1/+1 counter on Barrenton Medic.
Resolve the Barrenton Medic ability from step 2, untapping it.
Repeat.

Flourishing Defenses

Barrenton Medic

Combos with "Mighty Mutanimals"