Mephidross Vampire, and Walking Ballista

Prerequisites

• Mephidross Vampire on the battlefield
• Walking Ballista on the battlefield
• Walking Ballista has at least two +1/+1 counters on it

Steps

1. Activate Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to any creature an opponent controls
2. Walking Ballista triggers, putting a +1/+1 counter on itself
3. Repeat as desired
4. You may also 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 any number of creatures opponents control
• Infinite damage to creatures