go to the trainer house. you may see him there.
Use a fire pokemon. That's what grass is weak against.
the same way to beat Pokemon PEARL!!!!
Yes you also need to beat diamond as well.
then you beat the game
beat him with a level 100
No. In Pokemon Diamond you can only catch Dialga once but you MIGHT have a second chance after beating the Pokemon League at least once.
beat the Pokemon league
make your grass type learn other moves
you can go after the legendary pokemon, or beat him again.
Figure it out.
get a girlfriend
No, but you must get to a certain point in Pokemon Diamond to transfer Pokemon from Pokemon Ruby.