No detail upon how to retrieve these Pokemon/Items will be given but this page is to inform you of secrets you can try your best to figure out.
POKEMON
Mewtwo
Mew
Celebi
Deoxys
Spiritomb
Shaymin's Sky Form
Giratina's Origin Form
Cut Rotom
Heat Rotom
Spin Rotom
Frost Rotom
Wash Rotom
ITEMS
Master Ball
Glacedia Flower
Arceus Plates
Rare Candy
OTHER
Male Move Tutor - Frenzy Plant, Blast Burn, Hydro Cannon
Female Move Tutor - Draco Meteor