you just catch and train alot of pokemon.Hope it helped!
Only through a Pokemon event
You can't
Beat The Elite 4
I caught one through the right gate of Hearthome City
There is no way to get through the maids without walk/run through walls, then you should be able to get in.
Manaphy is not originally in Pokemon Diamond. In order to obtain it (without an action replay) you must play all the way through the game Pokemon Ranger. In the last (extra) mission of the game you can acquire the manaphy egg and then send it over to Pokemon diamond.
Only through a Pokemon event is it possible.
Dialga
You can get a Cyndaquil without trading only by transfering it from a GBA Pokemon Game onto your Pokemon Diamond Version.
Through normal gameplay you can't but I'll tell you some ways to get a legitimate Lugia.In third generation,if you have the Mystic Ticket,(which you can't get anymore,except through cheating) you can capture it at Naval Rock,then you can migrate it to Pokemon Diamond through the Pal Park.You can also get one by trading with Pokemon XD Gale of Darkness.Purify Shadow Lugia by getting all of the "chambers" in the purification chamber to be a "best circle",then trade it to a third generation game,and migrate it to Pokemon Diamond.Lastly,you can capture one in Pokemon SoulSilver,through normal gameplay,or Pokemon HeartGold in the post-game storyline and trade it over to Pokemon Diamond. These are the real ways to get a Lugia in Pokemon Diamond.
If you have a r4 with Pokemon diamond on it click on cheat y look walk anywhere then walk through all the walls
no