dtgreene: 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.
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.
Venita: My personal favourite for things like that is Kingdra with the right moves.
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.