Pokemon Ruby/Sapphire

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Manectric:
Found on Route 118, or by evolving an Electrike.

[UryuuLovePokemon]

Masquerain: Learn Bubble Beam:
Catch a Surskit. Train it to level 25. It would normally evolve at level 22, but it will not learn Bubble Beam if allowed to evolve at that level. Then, let it evolve into Masquerain at level 25 (after Surskit learns Bubble Beam). At level 26, Masquerain will learn its first move, Gust. Therefore, it will not miss any moves before that.

[UryuuLovePokemon]

Mawile:
Can be found in the Granite Cave in Dewford Town, only in Pokemon Ruby
Can be caught in the Sky Pillar, Granite Cave, or in the desert withthe fierce sandstorm. This Pokemon does not evolve into anything and is only available in Pokemon Sapphire.

[UryuuLovePokemon]

Metagross:
Fly to Mossdeep City and go after the Beldum (level 5) in Steven's house, after defeating the Pokemon League. Train it up to level 20 to evolve it into Metang, then up to level 45 to evolve it into Metagross. Note: Beldum onlyhas Take Down as a move until it evolves.
Give Metagross Meteor Mash, Earthquake, Shadow Ball, and Light Screen. Meteor Mash, Shadow Ball, and Earthquake allow for lots of damage, and Light Screen increases its weak special defense. You will have to use TMs to get Earthquake and Shadow Ball.
To get Metagross' first evolution, first get all eight badges. Then, defeat the Elite Four and the champion. Then, return to Mossdeep City and go to the to top left house. You should see a Pokeball witch contains Beldum.
To teach Metagross the move Meteor Mash, simply raise its level to 57.

[UryuuLovePokemon]

Metagross:
Fly to Mossdeep City and go after the Beldum (level 5) in Steven's house, after defeating the Pokemon League. Train it up to level 20 to evolve it into Metang, then up to level 45 to evolve it into Metagross. Note: Beldum onlyhas Take Down as a move until it evolves.
Give Metagross Meteor Mash, Earthquake, Shadow Ball, and Light Screen. Meteor Mash, Shadow Ball, and Earthquake allow for lots of damage, and Light Screen increases its weak special defense. You will have to use TMs to get Earthquake and Shadow Ball.
To get Metagross' first evolution, first get all eight badges. Then, defeat the Elite Four and the champion. Then, return to Mossdeep City and go to the to top left house. You should see a Pokeball witch contains Beldum.
To teach Metagross the move Meteor Mash, simply raise its level to 57.

[UryuuLovePokemon]

Mightyena:
Catch a Poocheyena and evolve it. Only available in Pokemon Sapphire

[Shiny Zoroark96]

How to get Shedinja:
Shedinja itself is a pretty weird Pokemon. It only has 1 HP, but (thanks to it's Characteristic- Wonder Guard) only attacks that are Super Effective against it will harm it. This means that unless it is against a Pokemon that knows a Fire, Flying, Rock, Ground or Dark move it is invincible, which depending on the Opponents Pokemon, could be pretty useful in your team, providing you return it at the right times.

Shedinja is the evolve of Nincada. What? You scream. But my Nincada (if you bothered you bothered to train one up) evolved into a Ninjask! How can I get a Shedinja now?

The answer is to train up a new Nincada. Make sure that you only have 5 or less Pokemon in your party and a Pokeball of some kind in your backpack. Then train your Nincada up to Level 20, and it will evolve into Ninjask. But what's this? Something has filled the empty space in my party! Why, it's a Shedinja!

Shedinja appears when you evolve Nincada into Ninjask, providing that there is a Pokeball of some sort in your backpack and a space in your party, you will get Shedinja. The way the Pokedex explains this is that when Nincada evolves into Ninjask, it leaves it's original shell behind. This shell is supposed to contain some sort of ghost, which is why Shedinja is part Ghost type. Yes its unethical, no it doesn't