you have to beat the elite 4 and the if you go to marvelous bridge the shadow triad will com and give you the griseous orb for giratina an adamant orb for dialga and a lustrous orb for palkia
In Pokemon Black & White or any other Pokemon, you can catch water type Pokemon in the sea or by using a fishing rod.
it is impossible without using the download from other games thing
you can't get rayquaza in Pokemon black/white. you can get it using gts negotiations , you can trade it with your friend or transferring from other version with poketransfer
You must defeat wild pokemon or pokemon owned by other trainers another way to level them up is by using the item rare candy.
Yes you can but you can't trade from black to pearl or white to diamond because the games are not compatible with each other because the Pokemon in black and white will come up as ? in the pokedex's of the other games. But you just can't trade to pearl from black or white sorry. However you can trade between black and white using infrared connection. (the other games don't have infrared that's why the game cancels the trade between pearl and black or diamond to black).
in Pokemon black munna evolves using the moon stone but be careful because it wont learn any moves after you evolve it
No, but you can transfer from platinum to black or white using poketransfer of relocater.
No, there is no data of Pokemon black or white in Pokemon platinum so it won't know about any of the pokemon.
by fishing or using surf
You can't, because Pokemon black was made after Pokemon diamond, Pokemon diamond wouldn't be able to recognise any of the new Pokemon. So to solve this problem they stopped all trading from black to any other game apart from white. You can however get your Pokemon from diamond to black by using the poke transfer, yet you need to have won the league for the first time to be able to do this.
No, but if you can link to someone who has Pokemon Black via wireless, you can visit his or her Black City using the Entralink.
Using a diddo.