Sengir, the Dark Baron, Agatha's Soul Cauldron, and Walking Ballista

Prerequisites

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

Steps

1. Give Sengir deathtouch, if needed
2. Activate Sengir using Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to any creature an opponent controls, and killing it due to Sengir's deathtouch
3. When the creature dies, Sengir's first ability triggers, putting two +1/+1 counters on it
4. Repeat from step 2 as desired

Results

• Destroy all creatures that enter the battlefield under an opponent's control
• Destroy all creatures opponents control