Walking Ballista in exile (exiled by Agatha's Soul Cauldron).
Yahenni and Agatha's Soul Cauldron on the battlefield.
Yahenni has at least one +1/+1 counter on it.
You have a way to give Yahenni deathtouch.
Steps
Give Yahenni deathtouch, if needed.
Activate Yahenni using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to a creature an opponent controls, and destroying it due to Yahenni's deathtouch.
Yahenni triggers, putting a +1/+1 counter on it.
Repeat from step 2 as desired.
If you can give Yahenni deathtouch indefinitely, you may repeat this loop whenever a creature enters the battlefield under an opponent's control.
Results
Destroy all creatures opponents control.
Destroy any creature that enters the battlefield under an opponent's control.