Mikaeus, the Unhallowed + Spawning Pit + Glen Elendra Guardian

Mikaeus, the Unhallowed

Spawning Pit

Glen Elendra Guardian

How it works

1. Activate Spawning Pit by sacrificing Glen Elendra Guardian.
2. When Glen Elendra Guardian dies, its undying ability triggers, returning it from your graveyard to the battlefield without a +1/+1 counter on it due to its replacement effect.
3. Resolve the Spawning Pit ability
4. Repeat.