Everything You Need To Know!:

To catch Deoxys: At the South-eastern corner of the third island map, there is a small
island with Deoxys on it.
There will be a triangle puzzle you need to solve to get to Deoxys.
against Deoxys. If you press the triangle, it will move randomly. Now
you need to get to the triangle and press against it in as few steps as
possible. If you did not do this properly, the triangle will move back to
its original position. If you do the puzzle correctly, it will glow hot
gradually. Once you've move the triangle enough, the ground will shake,
and the triangle will be shattered into pieces. Soon you'll see Deoxys
descending down the sky.
Deoxys will be at level 30. It has different sprites in the different
versions of the game if you managed to capture it.
• Mystery Gift
Go into any Pokemon Center and go to the Clipboard by the Shop Owner. Go to it and you can put in 4 words. Put in the words "Link Together With All." The Shopkeeper will notice this and give you access to the Mystery Gift system. Now save your game and reset and when you go to re-load it will be right below "New Game."

Legendary Pokemon
•Entei, Suicune, or Raikou will appear in the wild on the lands of Kanto after you have beaten the Elite Four and become Champion. However, only one will appear according to your starter pokemon:

•Squirtle - Raikou
•Bulbasaur - Entei
•Charmander – Suicune


Moves
•Explosion: Obtained from a Hiker on the Northern area of 1 Island.
P.S Only Ghost and Dark Pokemon can learn it.


•Elemental Attacks (Blast Burn, Frenzy Plant, and Hydro Cannon): Obtained from a move tutor in 2 Island. However, he will only teach your starter one of these attacks according to its type.


•Swords Dance: Obtained from a man located on the pier of 7 Island.

•Body Slam: Obtained from a tutor on 4 Island.