Anti-Venom, Horrifying Healer + Agatha's Soul Cauldron + City on Fire + Walking Ballista

Anti-Venom, Horrifying Healer

Agatha's Soul Cauldron

City on Fire

Walking Ballista

How it works

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

This loop relies on Agatha's Soul Cauldron granting the activated abilities of Walking Ballista to Anti-Venom, Horrifying Healer, turning any creature with a +1/+1 counter into a deadly turret. By removing a counter to deal 1 damage to Anti-Venom, its own replacement effect combined with the damage-tripling amplification of City on Fire causes zero damage to be dealt while crowning Anti-Venom with three fresh +1/+1 counters. This creates a net gain of counters that can be endlessly converted into lethal damage directed at any target.