team

Here's a team I used to beat the Elite Four:

Skarmory
- Spikes/Stealth Rock
- Roost
- Whirlwind
- Steel Wing/Aerial Ace

If you have access to the Stealth Rock TM at this point, that's the preferred move here. Stealth Rock will take off at least 25% for all of Lance's Pokemon since they all have a Flying type. Charizard's unlucky enough to take 50% damage. In fact, if you lead off against Lance with Skarmory, you can just keep spamming Whirlwind after putting Stealth Rock out and you could potentially KO Charizard before he even gets a chance to move. Roost helps recover any HP Skarmory's lost. Its excellent physical defense makes it a great way to switch in to something you know is going to have mostly physical attacks in order to heal up the rest of your team if you need to. Fire Blasts and Thunders from Dragonite are still going to hurt, so be careful leaving Skarmory out too long if you don't pull out Charizard with Whirlwind. Remember, Whirlwind always moves last. It may be a safer bet to let Jolteon start off, but the free damage from Stealth Rock is too good to really pass up even if Skarmory goes down in two hits.

Weavile @ Razor Claw (Sneasel works as well if you can't/haven't evolved it yet)
- Swords Dance
- Night Slash
- Ice Punch
- Brick Break

With Will being a Psychic type user, Weavile makes for a great lead. Ice Punch makes quick work of his Xatus and even comes in handy against Lance, while Night Slash will handle everything else. Brick Break is useful against Karen's Dark types, saving you from having to hunt down a dedicated Fighting type. Watch out for Houndoom!

Gyarados
- Dragon Dance
- Waterfall
- Earthquake
- Bounce

With Bounce, Waterfall, and Earthquake, Gyarados is your all in one counter to Bruno while still being able to be a massive threat to Karen and Koga. He's also useful against Lance's Charizard and Aerodactyl.

Typhlosion
- Sunny Day
- Fire Blast/Flamethrower
- Solarbeam
- Focus Blast

Typhlosion is almost able to take care of Koga on his own. He'll run into trouble with Crobat, but that's nothing Weavile or Gyarados can't fix. Fire Blast is the more powerful move, but some people prefer the higher accuracy and higher PP of Flamethrower, especially for in-game. Use whichever you're comfortable with.

Dragonite
- Dragon Dance
- Dragon Rush/Dragon Claw
- Extremespeed
- Earthquake

Dragonite will be your main counter to Lance and his Dragonites, with Weavile being your backup. The Dragon Rush/Dragon Claw option is the same as Fire Blast/Flamethrower for Typhlosion. Pick one and run with it. Extremespeed will let you outrun Koga's Crobat right off the bat if you don't want to build up a few Dragon Dances first, and will be working off Dragonite's 134 base attack which is going to hurt.

Jolteon
- Agility
- Thunderbolt
- Toxic
- Baton Pass

Jolteon makes a good annoyer with Toxic and is an obligatory Electric type. It makes for a good backup/setup for Weavile against Will with Agility and Baton Pass. Koga's Crobat won't be much trouble. Lance's Gyarados will be down from a single Thunderbolt, while his Charizard and Aerodactyl will also take some pretty serious damage. If Jolteon's got two or three Agility uses under its belt when the next Pokemon comes out, throw out a quick Toxic and then use Baton Pass to bring out your counter so all that extra speed doesn't go to waste. Dragonite and Gyarados are already offensive powerhouses without Dragon Dance. Free speed boosts will let them start tearing the Elite Four apart right away.

Sneasel works just as well and there isn't a huge difference between stats.