When Castiel absorbed all of the souls of purgatory, he becomes pretty much invincible:
- An Archangel's blade doesn't work on him (Sam pick's up Raphael's blade, and after the season 5 episode "Hammer of the Gods", a clear distinction was noted between an Angel's blade and an Archangel's blade, with the latter being the more powerful of the two)
He was able to prevent Raphael from teleporting with a simple thought (he didn't even need to click his fingers)
He managed to kill a countless number of angels in heaven without any effort
- He had the power to completely get rid of hell but chose not too
In season 11 episode 22, we see the Darkness easily weakened by:
Some magic from four or five witches combined (this seemed like a huge stretch. Even though the darkness was able to kill them, I don't see how that lightning-magic was able to harm her)
A few dozen demons that manage to lift her into the air(I can't imagine this ever happening to God-Cas based on what we saw of him. If he managed to kill thousands of angels, what's a few demons to him?)
An unknown long dagger-like weapon that Lucifer used that didn't seem to have any special properties and could've been used by anybody less powerful than him (an Archangel's blade didn't work on God-Cas, so I struggle to see how this could've done anything to him)
An Angel nuke (this one seemed slightly plausible, but then again, if God-Cas managed to kill all of those angels with what must have been a click of his fingers- I doubt he smited each one individually- then one would think the Darkness could have too, even from Earth).
It's hinted that God-Cas was on par with Michael, as he was afraid to get rid of hell as it would mean freeing him from the cage. Now since God is way more powerful than Michael, and the Darkness seemed to be at the very least on par with God, then it doesn't make sense as to why she seemed so vulnerable when God-Cas didn't have any weaknesses except for his his own power source (the souls).
So what is the explanation behind this? Inconsistency? Personally I think all high-order beings had their powers dumbed down this season, as we didn't get much "Archangel mojo" (the sort we saw from Gabriel) from Lucifer either.