After reaching the Grand Ascension Stage, one was essentially half a True Immortal, and through their cultivation, they could grasp some extremely powerful laws of heaven and earth, making them virtually completely invincible to Body Integration Stage beings. As for the final Tribulation Transcendence Stage, that was actually only an arbitrary division. Aside from a slight change in the nature of one's magic power, beings who reached that stage wouldn't actually be a whole lot more powerful than Grand Ascension Stage beings.
Furthermore, after reaching the Tribulation Transcendence Stage, there were no longer any early, mid, or late classifications. Most of them would also go into indefinite seclusion to try and transcend their true immortal tribulations in an attempt to ascend to that higher realm.
As such, Grand Ascension Stage beings were the most powerful beings in the Spirit Realm aside from those true spirit beings.