Infinite Damage with Walking Ballista and Aveline

Step 1: Have Walking Ballista on the battlefield with at least two +1/+1 counters on it.

Step 2: Use Vraska, Swarm Eminence's ability to proliferate and add an additional +1/+1 counter on Walking Ballista, ensuring it has enough counters to remove.

Step 3: Remove a counter from Walking Ballista to deal 1 damage to any target.

Step 4: Use Aveline de Grandpré's ability or any other green or black mana source to add a counter back to Walking Ballista.

Step 5: Repeat Steps 3 and 4 to deal infinite damage to any target.