Yes you can battle the elite four and the champion as many times as you want
Sorry but no. Maybe they will have a fight with N in Pokemon BLACK WHITE 2
You don't.
fighting and bug types
Flying and psycic
In Black and White Versions: Reshiram, Zekrom, Cobalion, Terrakion, Virizion, Kyurem.
before you last fight with N
You can fight Cynthia in black or white after you beat the elite four and ask for him/her at GTS.
If you are playing the first black and white: Dark,Fight,Psychic and Ghost pokemon!! In black and White 2 i don't know
In Driftveil City, you can fight a "heartbreaker" biker. It depends on the version, in Black, he'll teach you about Rotation Battles; in White, he'll teach you about triple battles.
When you finally manage to fight him, his Pokemon will be levels 75 to level 77. But after beating the elite for twice to get to him you should be fine.
You can also go to the nimbasa city and there in both the stadiums many people fight with you and they also have level 69 Pokemon
In order to fight Cilian (dent) you will need to start with Oshawott.