Vigor + Walking Ballista + City on Fire

Vigor

Walking Ballista

City on Fire

How it works

1. Activate Walking Ballista's second ability by removing a +1/+1 counter from it, putting three +1/+1 counters on it due to Vigor.
2. Activate Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 3 damage to any target.
3. Repeat as desired.