There is a creature with flying on the battlefield.
Steps
Activate Aerie Ouphes by sacrificing it, targeting any creature with flying.
When Aerie Ouphes dies, its persist ability triggers, returning it from your graveyard to the battlefield with no -1/-1 counter on it due to Grumgully the Generous.
Holding priority, repeat from step 1.
Resolve all Aerie Ouphes abilities, dealing infinite damage to the targeted creature(s).