It's not possible to find Sudowoodo in Pokemon FireRed. You will need to trade for one from a Pokemon Emerald game-save.
You can trade it from Pokemon Emerald. In Pokemon Emerald, when you find Rayquaza, there is a small chance that it is shiny when you find it. Save the game right before you fight him and keep trying until it turns black. Then catch it and trade it to Pokemon Diamond.
have a gba cable,go to the top floor,go to third counter,push "trade center" and have someone else with Pokemon Emerald,Ruby,Sapphire then trade. "but will save before entering."
you have to save at a Pokemon center in your gba game and then trade over Pokemon.
save again by the way the game lies
After you beat the Elite 4 go to Pal Park and save. Insert a Pokemon Emerald game and on the menu it says "Migrate from Emerald" go there. It shows all the PC Pokemon from Emerald. Choose 6 Pokemon and try to catch them!
start a new game and save it at the 1st oppertunity u get.
It saves automatically you can watch the ending or you can skip it by turning the game off.
You need the National Dex before you can migrate. Go to Pal Park, save the game, get the Pokemon you want to migrate in the PC, save again, play Platinum and at the menu it will say migrate from Emerald. Im the Pokemon expert. you are very lucky.
Save your game also have a strong water type Pokemon with you and a Pokemon that can make groudon sleep.
First you start a new game and then just save over your old saved game.
This will only work if you have 2 games (fire red and emerald)/ game-boys with adapters. Give the masterball to a high level (optional) Pokemon and trade it to emerald. In emerald go to the battle tower and put your Pokemon with the masterball in the PC and save. Next take the masterball from the Pokemon (if its a high level Pokemon withdraw the Pokemon and item). Now go to the first woman at the desk and choose open level, select to eligible Pokemon. When she asks if you want to save hit yes and when you see "saving don't turn off" shut off your game. Now turn it back on and don't worry when it says "corrupt file..." Go to your PC and see the Pokemon with the masterball then look in your party and see that same Pokemon with the masterball. Then just trade back and you have one in each game and 2 masterballs.