Both battles on the Steps of Faith (Vshap and make more sense in the context of the revelations during Heavensward's story, namely the whole cause of the Dragonsong War in the first place -
Nidhogg was fully capable of simply flying in and wiping Ishgard completely off the map if he ever wanted to. But the very fact he did not was the first clue there was so much more to the whole war than met the eye. Because Ishgard's complete destruction was not what he desired, no, total annihilation was too good for them he figured. He wanted them to endlessly suffer, to writhe and squirm in agony like a worm on a fishhook for eternity, to feel the pain he and all the dragons felt with Ratatsokr's murder.
So he was playing with Ishgard, inflicting just enough damage and chaos to wound them severely but still leave just enough to allow Ishgard to survive in suffering, when he would withdraw and go into slumber. And yes, that meant actually giving the Ishgardians a fighting chance, so the dragons flying in and attacking on the bridge where they could be more easily engaged by Ishgard's forces does make sense in a sinister way.
It's one of those situations that make more sense when taken in context.