People are massively forgetting something, Magnus uses the powers of Greater/Higher beings in the Warp right?? Arceus in this term would also have the creation powers that Magnus would have in the warp since the warp is basically just a dreamland, Tzeech represents change and his opposite is Nurgle, Entropy. By this logic, Magnus shares the same Weakness to Entropy and static decay as does his master. Arceus without a second thought would destroy the Universe if they wanted to, And we all know this from his fight against all three of the Space-Time, Gravity/Anti-matter Deities have he ignored them for an extended amount of time, defeated all of them without a second though and only after Time Plot armour level BS had Ash and Co. finally find a way to calm him down, not destroy mind you. Arceus is above a 4-D God like Dialga, World ending Yveltol and life-binding Xernes. The embodiments of the Soul, MiSPIRIT, Aziel and Uxie. Emotions, Will and Knowledge are all there above Dialga, Palkia and maybe Giratina, though this could be debated, are all under Arceus' control, meaning the very soul is in Arceus hands and Magnus kinda needs that to do anything, The Plates of Creation negates EVERY attack as shown by a Screenshot up there ^^^^^^^^^^^^ Protect, Detect, Counter, Perish song, Destiny bond, Light Screen, Glass Screen, Mimic, Metronome and all these are outs and wins for Arceus to countering everything and if per say doesn't come under Pokemon Attacks (But we all know it will) would be countered by these. Magnus punches Arceus, Arceus uses counter. Magnus uses an instakill attack, Protect/Detect/deflect. Magnus does immense damage to Arceus, Synthesis, Rest, Roost, etc. Magnus wants to get Healed, Heal Block. Magnus is about to kill Arceus' physical form, Destiny bond. There are far too many ways Arceus would win here. And if Arceus wants to, they could destroy the realm of Souls, the Warp, since it is basically a Dreamworld/Spirit world which are all said to exist which means they've already been created by Arceus and they could destroy them too.