Finding Dialga
It is impossible to miss the confrontation with Dialga. Unlike other legendary Pokemon in the game, Dialga is central to the story of "Pokemon Diamond." After obtaining the seventh gym badge at Snowpoint City and defeating Team Galactic at Veilstone City, you must ascend Mt. Coronet in search of Team Galactic's leader, Cyrus. Cyrus has taken control of the legendary Pokemon Dialga and seeks to use its power to take over the world. After you defeat Cyrus, Dialga goes on a rampage, at which point it is up to you to stop it.
Dialga
Dialga is a Level 47 Pokemon who is both Steel- and Dragon-type. As a result, Dialga does additional damage to Ice-, Rock-, and Dragon-type Pokemon. In addition, Dialga takes no damage from Poison-type moves and can not suffer any Poison-type status effects. Dialga is weak to Dragon-, Ground- and Fight-type moves. Dialga is not able to be bred with any other Pokemon and cannot be traded over the GTS, the "Pokemon Diamond" system for Wi-Fi trades across the world. Unlike other legendary Pokemon, Dialga does not run away when it is engaged. The fight ends when all of your Pokemon are defeated, Dialga is defeated or it is captured. Regardless of whether you defeat it or capture it, the story moves on and remains the same.
Fighting Dialga
If you choose to fight Dialga, rather than capture it, be prepared for a difficult fight. Dialga's Dragon-type moves, such as Roar of Time, causes large amounts of damage to any type but Steel-type Pokemon, so be prepared to heal your Pokemon with a Hyper Potion immediately after. Luckily, once Roar of Time is used, Dialga is not able to perform a move the following turn giving you time to heal. Once Dialga is defeated, the Pokemon who has defeated it receives 220 experience points. However, once Dialga is defeated, he disappears forever, as only one can be caught on each "Pokemon Diamond' cartridge. To get a Dialga after defeating it, you must trade for it locally with any friend who has captured one.
Catching Dialga
It is advisable to capture -- rather than to defeat -- Dialga due to its excellent move set and statistics superior to most other Pokemon. You are given a chance to save your game before fighting Dialga. It is a good idea to do so. If you defeat Dialga by accident, you can reload your save and attempt to capture it again. Status-inducing effects such as sleep or paralysis increase the chances of capturing Dialga, as does lowering its health bar until it is red. Use your strongest Pokeballs until Dialga is captured. Due to its status as a legendary creature, it generally takes more than one attempt to capture it.