Shay Cormac, Agatha's Soul Cauldron, and Walking Ballista

Prerequisites

• Shay Cormac on the battlefield
• Agatha's Soul Cauldron on the battlefield
• Walking Ballista in exile
• Shay Cormac has at least one +1/+1 counter on it
• You have a way to give Shay Cormac deathtouch

Steps

1. Give Shay Cormac deathtouch, if needed
2. Activate Shay Cormac using Walking Ballista's second ability by removing a +1/+1 counter from it, targeting a creature an opponent controls
3. Shay Cormac's first ability triggers, putting a bounty counter on the targeted creature
4. Resolve the Shay Cormac ability from step 2, dealing 1 damage to the creature and killing it due to Shay Cormac's deathtouch
5. When the creature dies, Shay Cormac's second ability triggers, putting two +1/+1 counters on it
6. Repeat from step 2
7. 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
• Destroy all creatures opponents control