Wow, I'm really surprised I haven't signed up for this one yet.
I'm doing water on Pearl. I'll start as soon as I get my team worked out. Done, and starting now.
Edit: -sigh- no water type can learn a fire move pre-national dex other than the banned palkia.
@Sudeki
Yeah, you pick a type, play through diamond, pearl, or platinum with only that type (dual types like infernape count towards both fire and fighting), and beat the elite four. You are allowed some HM slaves (because some types like fire don't have any avaliable Pokemon to learn some HM's like waterfall), but you aren't allowed to use these HM slaves in battles. You can use them in wild pokemon battles if you need to evolve them (like if you want a bibarel for surf, but bidoof can't learn surf). You are not allowed to use legendaries that fall under your type. You also have to have at least one Pokemon that is your type before you enter the Eterna Forest. Just use your starter until you get at least one Pokemon of your type.
Some challenges that I've done have allowed you to trade over like eggs of the type you're using, but I don't believe this is one of them. Just try to catch all the Pokemon in the wild, because it makes it more challenging and fun in my opinion.
For example, my water team is going to be Empoleon, Bibarel, Bibarel, Floatzel, Bibarel, and Starly. I plan on making the Starly learn fly and defog, and when I beat the game, he'll be still at like level 3. One of my bibarels that I'm not actually using in my team will learn Rock Smash, Cut, Strength, and Rock Climb. Empoleon and one Specbarel will learn surf, and Floatzel and Physbarel will learn waterfall (one bibarel is only going to learn physical moves, and the other is only going to know special moves, hence the names). All of these pokemon can be caught in the wild, and I won't need to trade to get any of them.