How & Where to catch/get - Dragonite w/ Multiscale (Hidden Grotto) in Pokemon Black 2 & White 2

Rarity - 1% [MOST RARE POKEMON IN GAME]
Method - Interact
Location - Route 18 - Hidden Grotto
Can you get this before Elite Four - No
Q) What is Hidden Grotto?
A) The Hidden Hollows are various small areas dotted around the Unova region. These areas are small gaps between two trees and can be accessed by interacting with the tree. While they seem relatively empty, they have numerous features which make them very prominent; The can give out Pokémon with Hidden Abilities, usually Pokémon from within that area. In addition to that, you may find a standard item sitting in the middle or an item hidden for access with the Dowsing Mchn.
Typically, when you go to an area, once you've mined it for its interactable Pokémon and items, it is useless, the Hidden Hollow is different. The contents of the Hidden Hollows change and get regenerated repeatedly.Whenever you take 256 steps in the game, there is a 5% chance that each of the Hidden Hollows will be regenerated. When it's regenerated, there's a 40% chance of an item, a 40% chance of a hidden item and a 20% chance of a Pokémon. The chance that it regenerates can be increased using a Pass Power.