Spawning Pit + Hardened Academic

Spawning Pit

Hardened Academic

How it works

1. Activate Spawning Pit 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. Resolve the Spawning Pit ability from step 1.
6. Repeat.