Okay, so I've been playing this game a lot recently. There is one thing that has crossed my mind multiple times.
Am I even going to be able to catch all the legendaries from the first four generations?!
So I've finally decided that I should probably ask before I get too far in.
Anyone can jump in and tell what legendaries they've found, anything from Mew to Arceus, I'm looking to see where they all are
and if they're even in the game.
So share your stories of conquest over the legendaries of the pokemon light platinum universe, lets get em all on here.

http://wesleyfg.weebly.com/legendary-pokeacutemon.html

It actually includes a lot of Gen V Legendaries. The only legendaries I notice aren't on the list are the Kamis, Victini, and Shaymin.

Awww..... no Victini? that's mah favorite pokemon... oh well the rest are all awesome too.

Actually that site is about a separate game on vizzed, called Pokemon Light Platinum. Its really all kind of confusing now that I've figured out the distinction, but the game that site is referring to is actually an updated version of the one I'm referring to. I believe it was made by the same developer, but Pokemon Platinum on vizzed, the first one, which is the one referenced in this post of mine, doesn't have the same list of legendaries.
Also, even though Shaymin is not listed on that website, he is actually found in the game Pokemon Light Platinum, he roams around Inhore Woods after you've got all the champions beaten.

i am not sure about the legendaries that exist in the game, but i can give you a very helpful tip on how to prepare to catch one.

first, have a number of Ultra balls: they give the best results.

second, when you finally find the legendary, try to take as much HP of his as possible, without killing him. for that, every time you have a lucky hit, quicksave (F2) and when you fail or kill him, go back (F4).

when you are happy with the amount of HP left, throw the ball, but just before you press a to throw it quicksave. then lean back, put your fingers the space bar (speed), the F4 button, and the a button and continue to going back to your quick save until you catch the pokemon.

this works, since every time you throw the ball, your chances of catching the pokemon are different. however, it may take time: it took me nearly an hour to catch Rayquaza. actually, it works with any pockemon, just use regular balls.

i call this the "one ball method", since it only uses one ball in the game.