Hello, I just want to share some idea of mine based on my walkthrough after beating Arceus.
In my point of view, Arceus is definitely weaker than Solaris at Pyrous Mountain. One of the reason is because Garchomp loves Earthquake and it'll kill all my pokemons. I'm sure there's a way to beat Solaris with mono poison. I just didn't look for it in this run unfortunately.
I noticed if Arceus doesn't do Judgement, he would pick Shadow Ball.
The key here is badly poison. So here's what I did.
Fortunately I just trained my Dragalge which have highest Sp. Def and she has Toxic. Dragalge is extremely slow and I regret training her speed. El had 1 Full Heal which he immediately used at first time he got poisoned. So do Toxic for 2 turn. And the rest is just survive!
This is my party after the battle
Pretty messy isn't it? All my pokemon is glass canon. But even this messy party could beat it. The only pokemon I have that can outspeed Arceus is Crobat (207 speed). And unfortunately I DID use some healing, 1 Ultra Potion.
So all you had to do, is bring one pokemon with Toxic moves, and others with high Sp. Def. A muk that can minimize might be a great idea! The probability he unleash the Judgement is 50/50 and before you get wiped out, he should be dead already.
Other idea is having Crobat learn egg moves, Hypnosis. But that would need a lot of time to breed it so... scratch that.
Have other idea?
Alright, onwards to Radomus Castle!