Stalwart Successor + Twilight Shepherd + Yahenni, Undying Partisan

Stalwart Successor

Twilight Shepherd

Yahenni, Undying Partisan

How it works

1. Activate Yahenni by sacrificing Twilight Shepherd
2. When Twilight Shepherd dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. When Twilight Shepherd enters, Stalwart Successor triggers, putting a +1/+1 counter on Twilight Shepherd
4. Resolve the Yahenni ability.
5. Repeat.