If gen 4 gets remade somewhere down the line like gen 3's going to later (being given the FR/LG/HG/SS treatment in other words), just wait for them to reveal a Mega Arceus... 😘
And I guess have a Mega Dialga and Palkia and stuff while they're at it since gen 4 ubers weren't overkill/over-the-top enough already.
I don't know about you guys but Mewtwo has slayed Arceus just fine for me in the past because Mewtwo had more outstanding stats rather than balanced. I always have Mewtwo with PsyStrike, Aura Sphere, Ice Beam and Thunderbolt. Lets him destroy Arceus no matter what plate he has (if he has Neutral damage I just psystrike which is insanely powerful with Mewtwo). And I also give Mewtwo NeverMelt Ice just because most other legendaries are dragon or flying. Unless the Arceus's EV are pumped in defence and Sp Defence I can usually destroy him in one (maybe two turns). However against people who know how to use Arceus properly I sometimes loose.