Well, for starters, I don't think Galbatorix's wards are 'conventional' durability. They're magic-based, and they're explicitly designed to block magic.
As far as that demon himself goes, baseline Galbatorix's magic is already above his tier. With his collection of Eldunari powering him up, he could completely obliterate him in a single attack. I don't know how potent the 'Magic Absorption' mentioned on his profile is, but unless he's absorbed attacks on-par with fully-amped Gal, I don't really see that working here either.
Also, I should have asked this earlier, but is outside help even allowed? Because Gal's dragon is restricted from use. (Though admittedly, Shruikan would be fodder here unless wards were placed on him as well.)