Uhhhh. I beat Ultra Necrozma on my 3rd try. Also, Ultra Necrozma would basically be guaranteed the win if it used Light That Burns the Sky (unless it's Arceus-Dark). To be fair, I never played D/P/Pt (Y was my first game), so I don't know how tough Arceus was. (Unless they nerf Arceus, I will know when BDSP is released...)
I still prefer Ultra Necrozma over Arceus.
Godzilla would obviously win, unless this was Ash's Pikachu. Ash's Pikachu destroys everything because plot device, so that seems kinda unfair.