Walking Ballista in exile (exiled by Agatha's Soul Cauldron).
Shay Cormac and Agatha's Soul Cauldron on the battlefield.
Shay Cormac has at least one +1/+1 counter on it.
You have a way to give Shay Cormac deathtouch.
Steps
Give Shay Cormac deathtouch, if needed.
Activate Shay Cormac using Walking Ballista's second ability by removing a +1/+1 counter from it, targeting a creature an opponent controls.
Shay Cormac's first ability triggers, putting a bounty counter on the targeted creature.
Resolve the Shay Cormac ability from step 2, dealing 1 damage to the creature and killing it due to Shay Cormac's deathtouch.
When the creature dies, Shay Cormac's second ability triggers, putting two +1/+1 counters on it.
Repeat from step 2.
If the method giving Shay Cormac deathtouch lasts indefinitely, you may repeat this loop whenever a creature enters the battlefield under an opponent's control.
Results
Destroy all creatures that enter the battlefield under an opponent's control.