Well, first, you would have to complete the SINNOH pokedex.(Diamond/Pearl)
You probably didn't know this, but:
In order to get Drifloon(Drifblim's first evolution form)... you need to go to Valley Windworks on FRIDAY (anyone's good, though).
And to get Lucario's first evolution form(Rialou)... Go to Iron Island. You can get here by making your way to Canalave City. Take a ship there... the ride's free. You will (eventually) meet Riley, a man who's afraid to go through the cave alone. You and Riley will battle some team Galactic Goons. Beat them, get him to the end, and he will show his thanks by giving you an egg. Make sure you have only 5/6 Pokemon in your party, or you will have to leave and redo getting to Riley.
These should be the only two Pokemon to obtain with a little difficulty.
Oncee you have completed this, and battled every trainer possible, your pokedex should be complete.
NOW>>> go to Sandgem city, where professor Rowan lives. Show him your completed (150 Pokemon seen) pokedex. Talk to him, and he will congratulate you. Then, professor OAK will come inside and talk to Rowan. After that, Rowan/Oak will update your pokedex to the NATIONAL level. After the updating... go to the end of route 221. The Pal park will be open and you can go inside. At this point, you should save your game and turn it off. Insert a gameboy advance Pokemon game (Ruby, Sapphire, Emerald, Firered, Leafgreen) into the GBA (Slot-2 of the DS) cartridge holder. Turn the game back on. STILL, go to Pokemon Diamond/Pearl. Press START to go to the main menu.This time, a new selection on the menu will appear. It will say "migrate from (the gba cartridge you inserted)". You may only select 6 per day. Your gba computer, where your Pokemon are being held, will come up after you're asked if you want to migrate Pokemon form whichever game. Say yes. Pick your Pokemon. BE CAREFUL. You can't migrate Pokemon with HM moves that have been taught (Flash, Surf). Note: You can't return these Pokemon to their original GBA games.
I wish you all the very best luck. Happy catching!