Walking Ballista's ability by removing a +1/+1 counter from it, dealing 1 damage to a creature an opponent controls, and destroying it due to
Kraven's deathtouch.
Kraven triggers, causing you to draw a card and put a +1/+1 counter on it.
Repeat as desired.
Notes
You can repeat this loop whenever a creature enters the battlefield under an opponent's control. This loop will work as long as your library isn't empty.
Results
Destroy all creatures opponents control.
Destroy any creature that enters the battlefield under an opponent's control.