Water-, Grass- or Fighting-type Pokémon will easily sweep his team. If you didn’t choose Piplup or Turtwig in the beginning of the game, you can catch Budew on Route 204 or Buizel or Shellos on Route 205.
Grass type gym leader located in Eterna City. Awards the Forest Badge.
Bug-, Fire- or Flying-type Pokemon will have the easiest time defeating her. Kricketot and Starly are commonly available before reaching her gym. Route 206 is also a great place to find Pokémon such as Ponyta and Zubat which can prove useful in her gym.
Fighting-type gym leader located in Veilstone City. Awards the Cobble Badge.
Use Grass- or Electric-type Pokémon to decimate Crasher Wake. Pachirisu (Route 205 or Valley Windworks) will take care of Gyarados and Floatzel, while any Grass-type Pokémon will be able to take care of Quagsire.
Ghost type gym leader located in Hearthome City. Awards the Relic Badge.
Dark-type Pokémon will easily be able to overwhelm Fantina’s team, but unfortunately there are next to no options at the point in the game in which you face her. Instead, we recommend training a Shinx to learn Bite and being sufficiently high enough in level to wipe her team. The Turtwig line also learns Bite.
Steel type gym leader located in Canalave City. He awards the Mine Badge.
Byron’s team makes it hard to choose a single Pokémon that can sweep each member of his team. Luckily, there’s one that fits the bill: Gastrodon (found as Shellos on Routes 205 & 218). If you raise a Shellos high enough to learn Earth Power (Lv. 35) or evolve it and have Gastrodon learn it (Lv. 39), you’ll have a powerhouse that won’t have a problem facing part Rock-type Pokémon (that would destroy Fire-types) or part Psychic-type Pokémon (that would decimate Fighting-types).
Ice-type gym leader located in Snowpoint City. She will give players the Icicle Badge after being defeated.
A Ground-type Pokémon will clearly be your best route to victory. Gastrodon, Hippowdon (found as Hippopotas, a rare spawn in Ruin Maniac Cave) or Graveler/Golem will be good allies for this fight.
The Elite 4 and Champion of Sinnoh are not to be taken lightly. Since you cannot switch Pokémon while challenging them, we will only give general advice for defeating them. We recommend leveling your Pokémon to at least 60 and carrying plenty of restorative items and revives so that you can take on each battle fully refreshed.
Fire-, Grass- and Water-type Pokémon will all prove very useful. Make sure that your team also knows Dark-, Bug- or Ghost-type attacks to take care of Lucian. Fighting- and Ice-type attacks can also be helpful. Good luck 😉