Step 1: First you need to have beaten all 8 gym leaders. Then you need any 2 pokemon level 30 with 4 empty spaces. Make sure that one knows fly.
Step 2: Fly to every city in the order that you got the badges. When you fly to the cities heal your 2 pokemon on every pokemon center.
Step 3: Go to the 7th island and catch an unknown
and keep it with you. Next, leave the Sevi islands, and go talk to your mom. When she heals your pokemon, talk to professor oak. He will check you pokedex.
The last step: Save the game and shut it off. After you've turned it back on, go to the nearest Pokemon Center and take out 4 pokemon. There's a specific list you have to have. When you press start to view your pokemon, you should have the level 30 flying pokemon first, the other level 30 second, a level 45 bug type third, a level 50 electric type fourth, and two others as your choice.
Now, go defeat the elite 4 using these 6. After you defeat them, instead of professor oak taking you to the hall of fame, you go straight to island 9 where you have to catch deoxys
in order to battle him for all 386 level 100 pokemon. But be warned, once on island 9 you can't save. So save before the battle with your rival!