Mikaeus, the Unhallowed | Spawning Pit (and one other card)

Persist Creature

Cards

Activate
Spawning Pit by sacrificing the creature with persist.
When that creature dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Resolve the
Spawning Pit ability.
Activate
Spawning Pit by sacrificing the Persist Creature.
When the Persist Creature dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Spawning Pit ability.
Repeat.

This combo has been estimated to be Exhibition (Bracket 1+).