Ashnod's Altar + Hardened Academic

Ashnod's Altar

Hardened Academic

How it works

1. Activate Ashnod's Altar by sacrificing the creature with persist.
2. When that creature 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 that creature.
4. Remove a +1/+1 counter and a -1/-1 counter from that creature as a state-based action.
5. Repeat.