Umbral Collar Zealot + Gravelgill Axeshark + Hardened Academic

Umbral Collar Zealot

Gravelgill Axeshark

Hardened Academic

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. Hardened Academic triggers, putting a +1/+1 counter on Gravelgill Axeshark.
4. Resolve the Umbral Collar Zealot ability.
5. Repeat.