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Well, when building a team, don't categorize Pokemon based on their type, categorize them based on their roles
For instance:
Special sweepers (attack with special attacks) – Alakazam, Gardevoir, Gengar, Jolteon, Sceptile, most Psychic types, Zapdos, etc.
Physical sweepers (attack with physical attacks) – Aerodactyl, Tyranitar, Salamence, most Fighting types, Tauros, Flygon, Dugtrio, Snorlax, Metagross etc.
Mixed sweepers (can use both special and physical attacks effectively) – Charizard, Dragonite, Salamence, Swampert are examples.
Special walls (defend against special attacks) – Snorlax, Blissey, Regice
Physical walls (defend against physical attacks) – Skarmory, Forretress, Weezing, Donphan, Cloyster, Steelix, maybe Metagross
Supporters (do things like heal the team, Rapid Spin, Baton Pass, phaze, and other things) – Celebi, Umbreon, Vaporeon, Blissey, Blastoise etc.
Good teams should have a physical wall, a special wall, at least one special sweeper, at least one physical sweeper, maybe a supporter.
A good team should also have no major weaknesses and at least one Pokemon to resist the following types: Normal, Flying, Ground, Water, Ice, Electric, Fighting, Psychic, Rock, and Fire.
So you're using a Sceptile. That means you've got Ground, Electric, and Water types covered as well as your first special sweeper.
Now you'll need a special wall, lets choose Regice since it can be found in Emerald. So now we have resistance to Ground, Water, Electric, Ice and special attacks in general since Regice is a special wall.
For a physical wall, lets choose Skarmory since it's the most popular one and can also be found in Emerald. Wow, Skarm has a lot of resistances. We're now resistant to Ground x2, Water, Electric, Ice, Flying, Normal, Psychic, special attacks in general, and physical attacks in general.
We could still use a proper Fighting, Rock, and Fire resist. Flygon resists Rock and Fire, Gardevoir resists Fighting, and both can be found in Emerald.
So we have Sceptile, Regice, Skarmory, Flygon, and Gardevoir. We can choose one more Pokemon, but make sure it isn't weak against Ice since we already have 2 Ice weaks and can't afford a third. It is preferable that this Pokemon be a physical attacker since we already have 3 special-oriented Pokemon. How about a Fighting type like Machamp or Heracross? That sounds like a nice, powerful addition.
Of course, you are free to choose which ever Pokemon you want, but each time you remove or add a new Pokemon, you gain new resistances but expose new weaknesses.
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