Destroy each creature that enters the battlefield under an opponent's controlDestroy any number of creatures opponents controlInfinite damage to creaturesLock
How it works
Activate Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to any creature an opponent controls.
Walking Ballista triggers, putting a +1/+1 counter on itself.
Repeat as desired.
You may also repeat this loop whenever a creature enters the battlefield under an opponent's control.