Mikaeus, the Unhallowed + Goblin Bombardment + Heartmender

Mikaeus, the Unhallowed

Goblin Bombardment

Heartmender

How it works

1. Activate Goblin Bombardment by sacrificing Heartmender.
2. When Heartmender dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Resolve the Goblin Bombardment ability.
4. Activate Goblin Bombardment by sacrificing Heartmender.
5. When Heartmender dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. Resolve the Goblin Bombardment ability.
7. Repeat.