Activate Walking Ballista by removing a +1/+1 counter on it, targeting itself, but putting two +1/+1 counters on it due to Vigor.
Activate Walking Ballista by removing a +1/+1 counter on it, dealing 1 damage to any target.
Repeat.

Infinite damage

Infinite +1/+1 counters on creatures you control

Vigor

Primal Vigor

Walking Ballista

Cast Gatta and Luzzu by paying {2}{W}.
When Gatta and Luzzu enters the battlefield, it triggers, targeting Walking Ballista.
Activate Walking Ballista by removing a +1/+1 counter from it, targeting itself, but instead putting two +1/+1 counters on it due to Gatta and Luzzu and Primal Vigor.
Activate Walking Ballista by removing a +1/+1 counter from it, dealing 1 damage to any target.
Repeat from step 3.

Infinite +1/+1 counters on a creature

Gatta and Luzzu

Activate Walking Ballista by removing a +1/+1 counter from it, dealing 1 damage to itself.
Rite of Passage triggers, putting two +1/+1 counters on Walking Ballista.
Repeat an arbitrarily large number of times.
Move to the next turn, removing all damage marked on Walking Ballista.
Activate Walking Ballista by removing a +1/+1 counter from it, dealing 1 damage to any target.
Repeat step 5 as desired.

Rite of Passage

Activate Anti-Venom using Walking Ballista's ability by removing a +1/+1 counter from it, targeting itself and putting two +1/+1 counters on it due to its replacement effect and Primal Vigor.
Activate Anti-Venom using Walking Ballista's ability by removing a +1/+1 counter from it, dealing 1 damage to any target.
Repeat.

Anti-Venom, Horrifying Healer

Agatha's Soul Cauldron

Activate Walking Ballista's first ability by removing a +1/+1 counter from it, dealing 1 damage to itself.
Druid's Call triggers, creating two 1/1 Squirrel creature tokens.
The Squirrels enters the battlefield, triggering Cathars' Crusade twice, putting four +1/+1 counters on each creature you control.
Repeat.
Once you have infinite +1/+1 counters on Walking Ballista, you may remove them to deal infinite damage to any target(s).

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