First things first , you can battle cheren once a day on Victory Road 7f. You can never see Ghetsis again and to find out about N,you have to do a little bit of legwork.let me explain.
You know when you beat the game and looker asks you to find the seven sages right? well you have to find all 7 sages(excluding Ghetsis) and when looker arrests the 6th(you can get them in any order) he will tell you about an eyewitnesses report about a person matching N's description and a Dragon pokemon somewhere very far away.
You also can battle Bianca anytime after you beat the game
contact me if you want to know the location s of the remaining 6 sages
HOPES THIS HELPS!!!