Pretty much he is saying that Galaxia should be able to affect Giygas.
To the battle itself, Giygas does infact starts with whatever he wants to, either OHKO, Status effects, Mind Manipulation or Just basic attacks. I highly doubt controlling his mind would do anything, but maybe his soul would, since at most it was stated that now he is made of "Evil Energy" or "Became Evil itself", so i guess he could still have a soul?
Counting what lamuna said, Galaxia can in attack Non-Corporeal beings, so I guess he's screwd there.
He's also above baseline by an Unknown amount, but I think someone stated that Galaxia was WAY above that, so I guess she stomps in AP.
Giygas existance erasure is tricky, since while he was eventually going to erase the kids if the battle would have prolonged more, he never seems to shown any of this during the fight, however, it is stated that he turned everything in the are into a void of Darkness, and during one of Paula's final prayer, it is stated that he was able to "Absorb her call into Darkness", so I guess he's in fact using his existance erasure like Void Manipulation during the entire fight and the kids are just resistance. I'll made something to discuss that later. For now, his existance erasure, as of what he has in his profile, is not reliable.
His attack reflection will help, since it was shown to work against beings in the same power has him, and compared to the shields of said being, he doesn't recive recoil damage, so he can shield itself against phisical attacks.
Anyway, I see Galaxia taking this more times due to actually showing veridic use of her strongest hax.