Did he fight Galactus in astral form?
Last I recall he TALKED to Galactus in Astral form.
But yeah Superman has lots of mental disciplines[Torquasm Vo Torquasm Rao and I think there was one or two more that weren't detailed much.] and is a formidable astral fighter.
When you say 'gone up against Galactus in Astral form' you give the impression that Xavier was actually facing off against big G.
He was not.
What he did is he went into astral form and tried to talk to and convince Galactus to spare the Skrull world. Xavier even tried to make Galactus feel all the goodness and potential the Skrulls had to offer.
However Galactus just told him it was for naught, and even had to HELP Xavier since his astal form had already started to deteriorate. Xavier would have perished had Galactus not helped to send him back to his body.
Xavier did not 'go up against Galactus in astral form' .....all he did is go to (not against) Galactus in astral form, try (in vain) to convince Galactus to leave the skrull world alone, and almost get destroyed in the process since he did not have enough energy to try and talk to Galactus and at the same time get back to his body.
All Xavier did is try (and fail) to convice Galactus, and almost destroy himself in the process. He needed help to get back to his body.