Mikaeus, the Unhallowed | Ashnod's Altar (and one other card)

Persist Creature

Cards

Activate
Ashnod's Altar by sacrificing the creature with persist, adding ({C} magic symbol) ({C} magic symbol) .
When that creature dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Activate
Ashnod's Altar by sacrificing the Persist Creature, adding ({C} magic symbol) ({C} magic symbol) .
When the Persist Creature dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Repeat.

This combo has been estimated to be Spicy (Bracket 3-4+).