Stalwart Successor + Umbral Collar Zealot + Gravelgill Axeshark

Stalwart Successor

Umbral Collar Zealot

Gravelgill Axeshark

How it works

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