Gen 1 really did have a lot of amusing bugs.

Eh, if you're going to transfer in a pokemon from another cartridge then any sufficiently high-leveled pokemon with a well-rounded move selection will do; Arceus is just plain overkill (not to mention that the only legitimate way to get one is from a Nintendo event; it's impossible to encounter without hacking your game).

Actually, there is a method to execute arbitrary code in Pokemon Emerald. It might be possible to get an Arceus (or any other pokemon) that way.
https://www.youtube.com/watch?v=m9pvNYdhldo

(No, not my video.)

Edit: Actually there is an error in my post (as astroclay pointed out); Arceus doesn't actually exist in Generation 3 games. (I am not as familiar with Gen 3 onward since I only played the Mystery Dungeon games.) On the other hand, getting Jirachi or Deoxys (both of which *do* exist in Platinum) might be possible via ACE.

In the first post, it sounds like deathknight1728 is playing pokemon emerald. If so, Arceus isn't obtainable. After all, it wasn't created until the fourth batch of pokemon games were released.

Therefore, the most powerful pokemon they could obtain would probably be Deoxys or Mewtwo. Of course, any legendary pokemon would really do for a solo run...

Unless there is a really powerful obtainable glitch pokemon, even if it's necessary to obtain it via arbitrary code execution and transfer it to the new file.

At a certain point, there's not much difference between exploiting a bug and just straight-up hacking the ROM. I'd say that crosses the line.

Sadly, Kingdra is available so late in the main storyline of Emerald that it's effectively a non-option. To catch a Horsea you need a super rod, which is only given to you towards the end of the game. However, the Horsea you catch isn't going to work since in Emerald version it never learns any dragon-type moves on its own! You'll need to breed a female horsea with an Alteria or Bagon in order to give their offspring the DragonBreath move. Once you have a Horsea that actually has the moves it needs you just need to level it up until it's a Seadra, find one of those rare dragon scales and trade it with the item equipped to trigger the evolution to Kingdra.

I do hate it when pokemon games do that, making a pokemon available early enough that you want to try and get it but so ridiculously hard to obtain that there's no point to even attempt it until the post-game. Still a great pokemon; the only type combos that resist both Dragon and Water are water/steel and grass/steel, neither of which exist in Emerald. Just too bad it's so impractical.