Infinite Damage with Walking Ballista and Vraska, Swarm's Eminence

Step 1: Have Walking Ballista on the battlefield with at least 2 +1/+1 counters.

Step 2: Use Vraska, Swarm's Eminence to give Walking Ballista deathtouch.

Step 3: Activate Walking Ballista's ability to remove a +1/+1 counter and deal damage to a target, destroying it due to deathtouch.

Step 4: Tap Gilder Bairn and pay to untap it and double the +1/+1 counters on Walking Ballista.

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