The Celestials are a supreme race of beings who stand 2,000 feet tall and are clad in armor that no one has seen them without. Their actions make them sound like your standard alien. You know, like they are so far beyond our species that they end up doing weird, evil things to us. For example, they've come to Earth to do experiments on people. That sounds pretty alien, right? They even created a sub-species of humans called the Deviants and the Eternals. Thanos is an Eternal, for reference, though he looks like a Deviant, which must be why he's so angry.
These are the creatures that Odin created the Destroyer to fight against. At 2,000 feet tall each, I don't think the Destroyer would stand much of a chance. Especially with Loki trying to steal it constantly.