Battle every trainer you see
There is only one legitimate way to complete the Sinnoh Pokedex. To complete the Sinnoh Pokedex you must trade for the missing Pokemon with Pokemon Diamond, Pearl or Platinum.
use the 100% complete pokedex cheat with the action replay(thats what i did).
A way to complete your sinnoh dex is to battle every trainer, Nintendo made it so if you battle every trainer you will complete your sinnoh dex.
trade it.
Feraligatr (Johto starter) evolve crocnaw which evolves from totodile (the only way to get a totdile is to complete the pokedex in emerald and talk to prof. birch)
1st you have to complete the sinnoh pokedex the go to professor Rowen and professor oak will come in and he will give you the national pokedex then go to the battle frontier and the people who are blocking the way will be gone I found out that on the 27th of April !!!!!!!!!!!!!!
the easiest way is to hack.
With a Budew.
erm you can't soree you'll just have to look up the missing Pokemon (their numbrs) on your pokedex and write them down then look up a trainer or route who has the Pokemon you need on the internet with Google and jus complete it that way that's how i did it it doesnt take long =-)
Congratulations on beating the champion in Pokemon Platinum! After that, you can explore the Battle Frontier, hunt for rare Pokemon, complete your Pokedex, participate in online battles, or challenge other trainers in the game for further enjoyment.
try transferring Pokemon back and forth from diamond, pearl, or platinum. that way, the pokedex in both games are filled.
Get an Action Replay and select 100% Pokemon on pokedex.