Catch Trainer Pokemon
Defeat a trainer and go to Seafoam Islands.Surf on the right side coast.You should run into a Pokemon from the trainer you just battled.
Talk to the man who tells you how to catch Pokemon.Answer "no" to his question.Put the item to be duplicated in the sixth place.Then,fly to Cinnabar Island and surf on the right side coast until you run into a Pokemon called Missingno or 'M'.Defeat or run from it to have your item duplicated.
Catch Safari Zone Pokemon Elsewhere
Go to the Safari Zone and walk until your time runs out.Go to Seafoam Islands and surf on the right side coast.You'll run into Safari Zone Pokemon,but you can fight them.
Raise Stats Fast
Battle with the Pokemon that's getting it's stats raised.Fight until all the PP (power points) of a move is used up.Next,deposit the Pokemon in a box.Then,withdraw it again.Check it's stats.It should show some improvement.
Hold "down" and hit "B" rapidly. Your damage will increase.
When you want to avoid a confused enemy's attacks, hit "A" and "B" together. Your enemy will hurt themselves.
Talk to the guy wo catches Weedle. Answer "no" the first time and "yes" the second time. Fly to Cinnabar Island. You'll find a Missingno at level 0. catch it and swim around a little more to encounter another Missingno. It will be Ditto. Release the first Missingno and withdraw Ditto. Ditto will be teh same as your 4th Pokemon.
Pokeball into Masterball
While ball is in midair, hold "B" and "u". When ball opens, hol "B" and "down". Hold until it stops moving.
Soothe Marowak's Ghost w/o Finding Silph Scope
When you come to Marowak's ghost in Lavender Town's tower, use a Pokedoll. Marowak's ghost will be soothed and you'll be able to pass.
Over Level 100 Pokemon
Talk to the man who catches the Weedle. Say "yes" first and "no" the second time. Fly to Cinnabar Island. Surf on half land, half water on the right hand side. You'll encounter Pokemon over level 100.