Agatha's Soul Cauldron, War Elemental, and Walking Ballista

Prerequisites

• Agatha's Soul Cauldron on the battlefield
• War Elemental on the battlefield
• Walking Ballista in exile
• War Elemental has at least one +1/+1 counter on it

Steps

1. Activate War Elemental using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to an opponent
2. War Elemental triggers, putting a +1/+1 counter on itself
3. Repeat

Results

• Infinite damage