The Thirty-Three Divine Beasts' Combined Magical Treasure was a complete set of Eighth Stage equipment.
Since it was a life-bound magical treasure, its strength and quality definitely surpassed those of ordinary magical treasures.
And because it was a combined magical treasure, its strength and quality were definitely improved even further after it was assembled.
Theoretically speaking, even if the 'Impregnable Holy City' was only at the Eighth Stage beginner level, as long as it was provided with enough spiritual energy, it would be able to defend against attacks at the peak of the Eighth Stage when in its combined state.
As long as there was enough spiritual energy, Lightning Palms of this level wouldn't be able to break through its defense.
Unfortunately, Song Shuhang's realm was only at the Fifth Stage, and even with his eight cores, he could only provide a limited amount of spiritual energy.