I don't know if this is possible but I am playing Pokémon Emerald and a thought occurred if I grinded a powerful dual-type creature and beat the game with it?

I am sure to be proven wrong, but I say - It cannot be done.
A single monster, even a duel-type is weak to several different attacks that would crop up in the E4 if you managed to avoid them everywhere else.
Try going through with all Eeveelutions, or just starters if you want a hard time. Trust me, a single monster (even 6 of them) will be toast at some point or another without any way of recovering.

Shedinja might be a help though. http://bulbapedia.bulbagarden.net/wiki/Shedinja

My thought is that it would have to be a certain type and require a lot of grinding. I mean I usually go with 3 man ptys so im sure some people have done it, they just probably grinded a shit ton.

I am playing Pkmon Emerald so would any of the starters or early pkmon be any good? I thought that I could use lotad because he is water/grass so he can destroy fire and water as well. I don't know how late he will get his 3rd trans though.

Bulbapedia and Serebii.net are going to be really useful if you want to go ahead with this.
Look at the early areas of Hoenn, the Pokémon available and see when they evolve and what you can teach them. Good luck.

http://bulbapedia.bulbagarden.net/wiki/Lotad
[url=http://bulbapedia.bulbagarden.net/wiki/Lotad_(Pokémon)/Generation_III_learnset#By_leveling_up]http://bulbapedia.bulbagarden.net/wiki/Lotad_(Pokémon)/Generation_III_learnset#By_leveling_up[/url]

http://bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_by_National_Pokédex_number
http://serebii.net/pokedex-rs/

I can't help but think back to Red/Blue where my 40th level plant type was using vine whip on everything. The screen kept saying 'not effective' but killing them in a single hit anyways...

Much like Nuzlockes and mono-type challenges, solo runs in pokemon are actually pretty common these days. The real difficulty is in the additional rules you may set for yourself. Will you limit the amount of potions you can use on your pokemon in battle? Will you be able to use other pokemon for HMs in order to pass obstacles in the field? Is it okay to breed a pokemon with perfect IVs for your initial partner pokemon, or do you rely on one that is more "natural" in its initial stat spread?

Regardless, I would suggest that before you start your solo-run in pokemon you carefully consider the kind of partner pokemon you want to use. You'll need to make sure it as a varied movepool, good stats, and a decent ability to handle most of the key opponents in your game (such as your rival, gym leader, and the elite four). You will also need to make sure that you have a good understanding of your pokemon's strengths and weaknesses. For example, is your pokemon a glass cannon that is physically oriented (if so, speed may be an issue), or do you have a bulky pokemon that relies on chip damage?

Remember, there are plenty of ways to play the game. The real challenge is in the limits and rules you set for yourself! :D

It should be doable since the highest level pokemon you would face is still less than Lv60. It would be a bit difficult at the beginning when the moveset is limited and the level advantage is not big enough. It shouldn't be a problem once you have significant level advantage. However, I think Emerald had a steeper difficulty curve than the games before it, so it would be even more challenging in the early game.

I've only played Yellow, Red and Crystal so my answer is completely based on that.

I'm going to tentatively say that I don't think you can solo them, at least not Crystal. Certain Gyms require certain HM moves like Surf and Strength to reach and off the top of my head I can't think of any Pokemon that can learn two or more HM moves. I could be wrong, but like I said that's just off the top of my head and I've not really played a Pokemon game in the past three or four years.

I am going to take the advice that it is possible. I will do a little research and see what I can do.

I wonder if Robopon, Medabots and Dragon Quest Monsters can do that too. That would be awesome!

But I came here for pet training games. :(

Blaziken is probably your best bet for the gen 3 starters, since it has decent speed, excellent attack, and a usable movepool. A moveset like Brick Break, Flamethrower, Earthquake, and Rock Slide would be good. Those moves have good accuracy and power, and good type coverage with each other. You'll be iffy against flying-types until you get Rock Slide, but after that point you should be able to handle everything.

Sceptile has a horrible movepool, learning almost nothing useful on its own and having poor TM compatibility; I really can't recommend it. Swampert is more defensively-oriented, and matches up poorly against the champion's team so he'd be a poor pick for soloing.

I'd have to run through the pokedex in detail to look for an ideal selection. A big problem is that I'm more thinking in terms of generation 6 rules, and there's a big difference between generation 3 and generation 6. Heck, I was this close to suggesting Gardevoir for a solo-run before remembering that it's largely powerless against dark-types in generation 3.

Generally the presumption is that you're allowed "slaves" in the back of your team who don't participate in battle but handle a variety of HM moves. Emerald has more than 4 required HM moves, so it would be outright impossible to do everything the game requires of you with a single team member.

However, there are a number of pokemon that can learn a large number of HM moves. Bibarel and Tropius are good examples of pokemon that can learn a full slate of HM moves and go a long way to reducing the load on the rest of your party.

Actually, in Yellow, it is possible, thanks to a glitch, to catch a level 1 Nidoking before the first gym. Get a little experience for Nidoking (not enough to reach level 2) and it will grow to level 100 (!), completely breaking the game. (Well, not as much as encountering Missingno., which is a gateway to other glitches including arbitrary code execution, at which point the game can clearly considered to have been torn apart to shreds.)

Arceus has 18 elemental forms, depending on which plate you equip it with. You should be able to solo with that, not counting utility pokemon.