Would love to see the first you played the game.
I never died to any of the Arch Demons (excuse the unavoidable cutscene at the start) sans the Old Monk, which during my playthrough, he had players posing as him and made a much more difficult and thrilling scenario. Maneaters, Penetrator and Old Hero were far more hard than "Press X to suicide" Astraea, "Hide behind pillars then press two buttons then smack a horn" Old God, "Spam the special attack" Storm King and "Watch me charging a very powerful attack while you attack me first and stunt me" Fake King Allant (admittely, this last combat holds some merit). Let's just not mention the real King Allant.
All in all, Demon's Souls is fairly balanced, but the majority of the bosses are an utter joke, fight-wise. It's disappointing that most of these happen to be the fabled Arch Demons, as well.
He is an easy boss because of those ballistas. Otherwise he is tremendously powerful as can be seen from in game descriptions.
1. God's Wrath
Miracle from the Demon Dragon God's Soul
Generates a strong blast of force around the caster.
This miracle represents the power of God against tremendous malice. It is one of the greatest miracles, aiding all who are righteous.
Spell from the Demon Dragon God's Soul
Generates a storm of fire around the caster.
A spell mimicking the raw power of the Dragon God, Sultan of Chaos. Its devastating flame is impossible to control.
Shoots a ball of fire and generates an explosion.
Born from dragon bones soaked in lava, this Demon was the embodiment of the power inherent in the colossal flames.
He is the only boss that can really kill you in 1 or 2 hits in NG regardless of your soul level. Those ballistas you fire are huge and he still survives 2 of them. He is also called a God and in all the Souls games Gods are the most powerful of their group so I do believe he is the strongest Dragon in any Souls game.
I'm not talking lore-wise, however, which is something I can agree with Dragon God. It's like Seath the Scaleless in Dark Souls, which shares many similarities to Dragon God. They both defeat the player at least once, they're the responsibles for very powerful magic (those spells in Demon's Souls, and Sorceries in Dark Souls), and they are both responsibles of massive cataclisms. In both their battle grounds, you can see the remnants of battles within (dead soldiers with the Dragon God, and cursed knights with Seath). Ultimately, however, they're vanquished by the player in a fairly easy combat. Dragon God goes down with two ballistas (otherwise he can't be damaged), and Seath goes mortal by shattering the crystal (otherwise he can't die).
I wanted an actual battle, vanquishing the giant foe, but that's not what you get, which is what I found disappointing. You get two cutscenes and then a mercy killing.