Walking Ballista and Vraska, Swarm's Eminence Infinite Damage

Step 1: Have Walking Ballista on the battlefield with at least one +1/+1 counter.

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

Step 3: Sacrifice a creature using Ghave, Guru of Spores and Ashnod's Altar to produce mana and a +1/+1 counter.

Step 4: Use the mana to add a +1/+1 counter back to Walking Ballista.

Step 5: Remove a counter from Walking Ballista to deal damage to a target creature.

Step 6: The target creature dies due to deathtouch, triggering Ghave to create a Saproling, which can be sacrificed for more mana.

Step 7: Repeat steps 3-6 for infinite damage.