well, the Named angel that spawns in the city is one of the hardest foes in the game, so you would have to be insanely strong, fast ,well armored, and highly trained in whatever weapon you are using.
Its probably not going to happen by polymorphing, as polymorphing has some pretty strong limitations on your ability to grow as a charachter. I think you should focus more at this point at finding a way to achieve one of the easier endings than trying to take on the cathedral.