Stalwart Successor + Umbral Collar Zealot + Heartmender

Stalwart Successor

Umbral Collar Zealot

Heartmender

How it works

1. Activate Umbral Collar Zealot 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. When Heartmender enters, Stalwart Successor triggers, putting a +1/+1 counter on Heartmender
4. Resolve the Umbral Collar Zealot ability.
5. Repeat.