Yahenni, Undying Partisan, Agatha's Soul Cauldron, and Walking Ballista

Prerequisites

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

Steps

1. Give Yahenni deathtouch, if needed
2. 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
3. Yahenni triggers, putting a +1/+1 counter on it
4. Repeat from step 2 as desired
5. 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