Walking Ballista + Mikaeus, the Unhallowed

Prerequisites

Walking Ballista on the battlefield with a +1/+1 counter on it.

Steps

1. Activate Walking Ballista's last ability by removing a +1/+1 counter from it, dealing 1 damage to itself.
2. Walking Ballista dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. Repeat.