To battle Blue in Crystal, you must first beat all of the Johto Gym Leaders.
After this, you must beat the Elite Four and then the first seven Kanto Gyms.
Go to Cinnabar Island (below Pallet Town) where you'll meet Blue.
He'll then leave for Verridian City, in the last gym.
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Yes you can link battle between Pokémon Blue and Pokémon Crystal however you can't use Johto-introduced Pokémon or Johto-introduced attacks though.
It is not available in Pokemon Crystal. You will need to trade for it from Pokemon Red, Blue, or Yellow.
Have a very srong pokemon
trade it from red,blue, or yellowMewtwo is not catchable in the wild in Pokemon Crystal.
First, you have to find them, then, you have to battle them, then, you catch them.