We are talking about Marvel's Celestials, said to be slightly weaker than Galactus in terms of cosmic power (average individual).
Batman has access to all the prep he gets as evidenced throughout the comic history - Justice League resources and up to 10 years planning, without the likes of his rogues' gallery annoying him. He starts the prep with a motherbox (For ease of transportation) and unlimited battery power (Magic supplied by the Lords of Order and Chaos).
However in the fight itself, he fights alone. Otherwise it would be "Justice League vs 1 average Celestial" instead.
The fight takes place across the entire galaxy, so the Celestial simply can't just blow up any planet that Batman is in.
P.S: If Batman wins with ease against a single Celestial, then assume that this battle has him fighting 20 celestials instead.