The codes below can be used when speaking to Munna in Easterly Town to get Pokemon at the level noted. Beat the Pokemon to get them:
Electric Up Pikachu (3-2 Volcanic Slope): 7580-5216
Fire Boost Tepig (2-2 Misty Edgewater): 5487-0833
Grass Boost Snivy (2-3 Soothing Shore): 4941-6440
Lugia (2-4 Shimmering Lake):
3467-3843
Victini (2-1 Sun-Dappled Bank):
9082-7838
Invisible KO Glitch
Select a pokemon with an attack that comes from the sky (Solorbeam, Thunderbolt, Brine, etc.) and go to Northerly Town. Leave town. Now go to the Battle Royale to the right BUT DON'T GO INSIDE! Hug the wall of the circular part of the building, and use your attack. It will say that you defeated a Cubchoo or Vanillite.
Legacy Mode
Having completed the game, hold L, are and Left (D-Pad) while on the title screen. With these buttons held, press A to start the game. You should then hear a noise.
This will activate Legacy Mode, resetting the environments to how they were at the start of the game. This lets you get low-power Pokemon regardless of your rank.
Reset game
While starting the game, hold A + B + X + Y. Release the buttons at the title screen, and a message will appear asking if you want to delete your data.
Secret (P) Spots
If you want to get some extra (P) there are spots across the maps where there looks like there isn't anything there. One spot is 2-4 at the lake. On the last part of the level, at the round spots at the far west and east parts.
Special Toys (North America)
Talk to Munna in Easterly Town and you'll have the option to enter codes to unlock Pokemon that you have defeated. The codes and the corresponding toy Pokemon are indicated below. Keep in mind that they only work with the North American edition of the game.
EU Special Toys...