Umbral Collar Zealot + Grazing Kelpie + Hardened Academic

Umbral Collar Zealot

Grazing Kelpie

Hardened Academic

How it works

1. Activate Umbral Collar Zealot by sacrificing Grazing Kelpie.
2. When Grazing Kelpie 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 Grazing Kelpie.
4. Resolve the Umbral Collar Zealot ability.
5. Repeat.