To be able to catch Zekrom (Black 2) or Reshiram (White 2), we must first head to N's Castle . Getting there is a bit more difficult than it was in Black and White, but it's still not too hard. The best way to reach it is to Fly to the Pokemon League then head into Victory Road from there
Starting from the Pokemon Center at the Pokemon League, head south until you're out of the area and back at Victory Road. Once you are, move around and enter the cave. Head east, then south to exit the cave.
Victory Road - Cliffside
Now head down the stairs past the two Veterans here. When you see a thin staircase leading down to your south-west, move down it. Move down the second one, and you'll be given a choice between two staircases to move down.
Take the eastern of the two and you'll see Zoroark here who will run when you reach the bottom of the stairs. Follow him through the Dark Grass into the cave.
Head west down the first set of stairs in the cave. Now head south until you reach the large set of stairs. Move down them, jump over the ledge and exit the cave at the bottom. You'll see Zoroark outside. Move over to him and he'll run into the next cave.
Victory Road - Cliffside Cave
Follow him in once more. Move up to him and he'll once more skitter off. Head north until you see the huge staircase leading down east. Move down it into N's Castle at the bottom.
As soon as you enter, you'll see N nearby. Move up to him and he'll talk with you. Answer 'Yes' when he asks his question and he'll team up with you. You can't explore the castle without him teaming up with you. Once he's travelling with you, head north through the door.
You'll be in a room full of toys. You'll talk with N some more, before he'll head back out of the room. Don't follow him just yet. Instead, head to the eastern side of the room and grab the Luxury Ball sitting there. Once you've done that, follow N out of the room.
You'll team back up once more. Head south down the stairs made of rubble. Move west and you'll find an item. Pick it up to find that it's some Protein . Now move east until you reach the next set of rubble stairs. Move up them and go east until you reach yet another set of rubble stairs leading down.
Move down them, then head east until you reach TM50 - Overheat at the end. Pick 'er up and head back up the rubble stairs. Now head east, then north and you'll reach a proper set of stairs. Head up them and go west until N says that you should continue alone. When he does, go through the door ahead.
Head north through this area until N calls out and catches up. You'll continue to talk before he throws out his Zekrom (Black 2) or Reshiram (White 2) and challenges you to a battle. This is his only Pokemon, so it won't be a crazily difficult battle.
Even so, if you don't think you're up to the task, you can answer with a 'No' and come back later. If you answer 'Yes', the battle will begin.
Trainer Battle: Pokemon Trainer N
If playing Black 2
If playing White 2
Note : N has a Full Restore
Zekrom and Reshiram are both tough, but it's 6 Pokemon versus 1, so you should manage. Getting good type match ups is hard because of their fairly unique typing. For Zekrom, Ground-types will have a distinct advantage, while Rock-types will work best for Reshiram.
Once N's Pokemon has been defeated, N'll talk with you, then turn it into a stone. In Black 2 this is the Dark Stone . In White 2 this is the Light Stone . N will then give this stone to you. Guess what? You can use this to catch the Pokemon you just battled!
If you haven't yet explored north-west Unova, head out of N's Castle and the cave, then Fly to Opelucid City , head west onto the Tubeline Bridge and begin exploring. If you already have, Fly straight to Icirrus City so that you can take on Dragonspiral Tower and catch us a Legendary.