Mikaeus, the Unhallowed + Heartmender + Yahenni, Undying Partisan

Mikaeus, the Unhallowed

Heartmender

Yahenni, Undying Partisan

How it works

1. Activate Yahenni 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 Yahenni ability.
4. Activate Yahenni 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 Yahenni ability.
7. Repeat.