Personally, I was under leveled and defeated N but got thrashed by Ghetsis so i would suggest that level 56 is the level you should train up to. Beware of his Hydreigon.
Have at least level 56 Pokemon, and make sure they are good.
Yes instead of fighting adler, you go to N's castle and fight him there. But don't worry, you still fight adler eventually. (and Cynthia for some strange reason. But the recommended level is 52, and for ghetsis, who you also have to fight the recommended level is 54.
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 beat ghetsis wich is n fathers name by getting pokemon level 70 or higher the best team to go with is latios black kyurem registeel salamence staraptor and genesect but remember level 70 or higher! i used level 90 i think thats the way to go if you want to beat him fast!
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
a easier way of leveling up is below! Pokemon level 1-30 find a level 50 Pokemon Pokemon level 31-100 find a level 100 Pokemon Pokemon level 101-200 fight a level 200 Pokemon Pokemon level 201-300 fight a level 300 Pokemon and so on... WARNING YOU MUST DO THIS ON VERY EASY!!
to level up your Pokemon walk through grass. eventually you will get attacked. fight and win. you will get exp. when a Pokemon gets enough exp, it will level up. for more info, press L or R on your DS/ game boy.
In my opinion, I always goto the elite four (aftergame) and replay them. You can also goto the east side of Unova and fight wild pokemon.
Klink evolves into Klang at level 38 on Pokemon Black.
futachimaru evolves at level 99 in Pokemon black and wight
krookodile evolves at level 40 in Pokemon black and white
Level 43