there is no way sorry
There are two ways of getting Groudon on Pokemon Diamond. The First way is trade it with people in the GTS (Global Trade Station). Or you can migrate it from Pokemon Emerald, Ruby, or Sapphire.
There is no way to trade between Emerald and Diamond, because they are for different systems. The closest thing is that if your friend has beaten the game then you can put your Emerald into his DS and put your Pokemon into his game. The only problem is that you can't trade Pokemon from Diamond to Emerald, only Emerald to Diamond. however there is a way to get the Pokemon from a GBA cartridge to the DS cartridge. when you get in to the Pal Park supposedly if you have a GBA cartridge in the slot2 and a DS Pokemon game in slot1 the Pokemon from the GBA cartridge will "migrate" to the slot1 game.
u can migrate Pokemon from fire red, leaf green, ruby, sapphire, and emerald to diamond, pearl, and platinum but not the other way around.
There's no possible way to trade back. Once you've transferred from Emerald, there's no going back. It's in Diamond forever, or at least until you delete your game.
the only way to get groudon is to either import it from Pokemon sapphire/ ruby/ or emerald, OR, trade it from pearl/diamond/or platinum.
You can't trade back to Pokemon pearl from emerald, the games aren't compatible that way. And no, Dawn stones didn't exist in leafgreen time.
Normally you can catch it in pearl after beating the elite 4 but in diamond the only way is to trade with someone who has a sealeo on their pearl, or migrate it from emerald.
There is no possible way to go underwater in Pokemon diamond. You could only go underwater in Pokemon ruby, sapphire and emerald. There is no possible way to go underwater in Pokemon diamond. You could only go underwater in Pokemon ruby, sapphire and emerald.
you don't get rayquazza in Pokemon diamond the only way to get rayquazza is to transfer one from Pokemon ruby Sapphire and Emerald to Pokemon diamond
trade...no other way
The only way to get Regice on pokemon diamond is to transfer them from Ruby, Emerald, or Sapphire.
The only way to get Hoppip on Pokemon Emerald is to trade from Fire Red or Leaf Green.