You get a lucky egg in about half way through the game. Also trading with someone.
take a weak pokemon to the elite four
Train them............
you train them
if you want to level up your pokemon faster. give the pokemon you want to level up an exp share but it at the bottom of your pokemon list and battle and beat the elite4 in succession over and over until it is at the level you want it to be.
Level 64
Level 43
Level 5
futachimaru evolves at level 99 in Pokemon black and wight
Klink evolves into Klang at level 38 on Pokemon Black.
krookodile evolves at level 40 in Pokemon black and white
Roggenrola evolves at level 25 into Boldore on Pokemon Black.
krookdile evolves at level 40 on both Pokemon black and white