Keep in mind, all the following is assuming Arceus can change his type at will. To start off, let's lay down some basic ideas about Arceus. He has a Base Stat Total of 720, which surpasses all Pokémon save for the Mega Mewtwos, Mega Rayquaza, and Primal Kyogre and Groudon. Its stat distribution is even across the board, so it is well-rounded and can be built in any way with EVs. It can change its type with its ability, Multitype, using the corresponding Plates. These plates also give a boost to attacks of that type. His signature move, Judgement, is a relatively powerful move (100 base power) that corresponds to Arceus's type. Therefore, it always gets STAB (same-type attack boost). He also boasts powerful moves such as Hyper Beam, Extreme Speed, Future Sight, and Recover. Since Recover recovers half his max HP, it'll be something important to note. Also, a stat-boosting move he has is Cosmic Power, which boosts his Defense and Special Defense by 1 stage. This can make him a tanky force to be reckoned with. Based on the moves available, I'll put his main moves at Cosmic Power, Extreme Speed, Judgement, and Recover to give him the most optimal moveset. HMs and TMs won't be included because they're human-based ways of giving him power. A thing to note is that his moveset is Twister, Judgement, Flamethrower, and Blizzard in the movie, but that's not reaching his full potential. Scenario 1: With no preparation, the Persona-users will have a tough time. Arceus's Blizzard from the movie will take out users like Yukiko with ease, and his constantly changing type gives him an advantage, as there are 18 types with different weaknesses and resistances. However, he is not immune to instant kills. But assuming he has instakill boss immunity, he'll be able to use his type changes to cause enough confusion to get at least one Cosmic Power up. However, the sheer numbers may be too much for him... After all, this is roughly 25-ish(?) Persona-users against one god. However, in the movie, he took the attacks of like 50 pokémon like a champ. Due to Cosmic Power bulk and Recover, Arceus can probably outlast everyone long enough to wreck everyone, especially with the use of Judgement. Actually, should he shift to Fairy-type, he'd have no exploitable weaknesses in most cases... but I digress. Although one thing against him is a potential Dekunda that can negate these bonuses... Scenario 2: The Persona-users have the advantage here. Same thing as last time, but they'll know about Multitype and have enough elements to strike each weakness. They'll also have more knowledge of which form has what weaknesses. With a good strategy, the Persona-users can reject a god! ...provided that they are prepared enough. Cosmic Power and Recover will still be an issue, after all. Not to mention his very strong attacks... Bonus: *shrug*