you can only get corsola with the swarm.
to know what and where the swarm is ask dawns little sister in sandgem town.
you can catch dittos in Pokemon diamond and pearl with poke radar in route 218 but its pretty hard to catch them + find them. :)
i would catch shinx or i would catch the opposite type of your rivals starter/ I Would catch a starly (die hard metallica fan)
If you want to catch a Pokémon called Apalmin you will have a hard time. This is because such a Pokémon does not exist in Pokemon Diamond.
choices: all if you train them hard enough * milotic * gyarados * lumineon * whishcash * empoleon * ...
you can catch it in route 212 ya but it is really hard must have a garbage pail
Skarmory is found on the path leading up to Stark Mountain. This is a hard catch, so pack some powerful balls and good pokemon.
not hard
RAYQUAZA
a lot hard
First, catch all of the Sinnoh Pokemon. Then you have a lot to do. You must transfer Pokemon from the GBA games, trade with Pearl if you have Diamond or with Diamond if you have Pearl, and breed Pokemon for Eggs. Completing the National Pokedex is very hard and will take a long time, but if you stay with it (and if you don't use cheats), It will give you something to brag about. May your Poke Balls always lock!
Find a wild Chansey, they can be found on route 209 near Solaceon town, and then use Covet, Theif, or another item stealing move, or else catch it, but they are hard to catch.
=you first have to complete the sinnoh dex,this is not that hard.==however the national dex is,YOU HAVE TO CATCH EVERY POKEMON THERE IS!!!!!!!!!!!!!!== you HAVE to trade and migrate,==hope it helped!==^_^=