Can batman really beat everyone with prep time?

No.

With Darkseid level Characters like Silver Surfer and The JLA, yes

with Galactus level Characters like Thanos, Living Tribunal, Anti-Monitor, maybe even Apocalypse, No

A thousand no's

Well, usually no one but the Black Cat.

Well, not everyone.

No. Not at all.

No, of course, he can't beat everyone with prep time. But he can take down rather powerful opponents. Darkseid and Superman for example. Now would he fair against extremely powerful dudes like The Living Tribunal or Lucifer Morningstar? No of course not.

He can't beat Galactus for example.

No