Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight | Shilgengar, Sire of Famine

Cards

All permanents on the battlefield (
Strangleroot Geist without a +1/+1 counter on it).

Activate
Shilgengar by sacrificing
Strangleroot Geist.
When
Strangleroot Geist dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Shilgengar ability.
Activate
Shilgengar by sacrificing
Strangleroot Geist.
When
Strangleroot Geist dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Strangleroot Geist enters and dies as a state-based action due to having 0 toughness. Its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Shilgengar ability from step 4.
Repeat from step 4.

If
Strangleroot Geist starts with a +1/+1 counter on it, then you may start the combo from step 4 instead.

This combo has been estimated to be Oddball (Bracket 2-3+).