(Sorry this post took so long! I was doing my own research and planning this all from scratch!)
Here's some suggestions from someone who's still learning competitive battling. Anyone's free to correct me, but I feel these are decent choices. Also, feel free to change things as you want. Have fun with it! (Especially if you're just battling friends. ;P)
If I use any words you don't understand, here's a really good site that explains competitive battling terms.
The Pokemon Dictionary
Charizard
This one will be difficult to use thanks to Stealth Rocks. As a Fire/Flying it's 4x damage which could easily bring him to half health just for switching him in. If you use him, he'd probably be best as a Special Sweeper.
Nature: Timid (+Spe, -Atk) or Modest (+SpA, -Atk)
EVs: 252 Speed, 252 Special Attack
Moveset:
- Air Slash
- Fire Blast
- Focus Blast
- Solar Beam
Other good options you can switch between depending on the type coverage needed. Hidden Power Grass (instead of Solar Beam), Flamethrower (instead of Fire Blast), and Dragon Pulse in place of anything you want. If you have Mega Charizard X, I'd take Dragon Pulse for sure.
Jolteon
I need more experience with Jolteon. I hear he's great and I know he's built to be a Special Sweeper. Here's what I would personally do with him, but feel free to make changes to his moveset. I had a hard time picking.
Nature: Modest (+SpA, -Atk)
EVs: 252 Speed, 252 Special Attack
Moveset:
- Volt Switch
- Thunderbolt
- Shadow Ball
- Substitute
Some more good options: Charge Beam with Baton Pass and Thunder with Rain Dance somewhere on the team. Might be more good moves too. Look at all he can learn yourself.
Mawile
Mawile is a good Bulky Physical Sweeper or he could be a good Baton Pass for another Physical Sweeper. If you go the Baton Pass way, then you would want to buff him up as much as you can and Baton Pass into Scrafty or Greninja. I'll give both of my suggested sets here.
Physical Sweeper
Nature: Adamant (+Atk, -SpA)
EVs: 252 Attack, 252 HP
Moveset:
- Iron Head
- Crunch
- Swords Dance
- Fire Fang, Thunder Fang, Ice Fang (take your pick)
Baton Pass
Nature: Impish (+Def, -SpA)
EVs: 252 HP, 252 Defense
Moveset:
- Baton Pass
- Swords Dance
- Substitute
- Iron Head
Alakazam
The original Special Sweeper. There's a lot of viable moves for Alakazam and you'll have to pick based on your play style. I'll give you what I would use and some options under that.
Nature: Timid (+Spe, -Atk) or Modest (+SpA, -Atk)
EVs: 252 Special Attack, 252 Speed
Moveset:
- Psychic
- Shadow Ball
- Focus Blast
- Substitute
Also try: Calm Mind, Psyshock, Energy Ball, Encore, Dazzling Gleam (Anti-Dragon!)
Scrafty
Yet another Bulky Physical Sweeper. His speed is pretty bad, but he's a fun guy to use. If you make Mawile a Baton Passer then Scrafty will become even better.
Nature: Adamant (+Atk, -SpA)
EVs: 252 Attack, 252 HP
Moveset:
- Drain Punch
- Crunch
- Ice Punch, Thunder Punch, Fire Punch
- Bulk Up
You can also use High Jump Kick or Brick Break in place of Drain Punch (since Drain Punch isn't easy to get if I remember correctly). If you have the Shed Skin ability and don't have Drain Punch, Rest would be good too. Dragon Dance can replace Bulk Up if you like too. Other moves to try include Dragon Claw, Dragon Tail, Iron Head, and Poison Jab (Anti-Fairy).
Greninja
I have no idea what to do with Greninja yet, honestly. I'd definitely make sure he has the Protean ability if you use him. Here's what I would try, but I'd suggest waiting for someone else to give you their advice on the matter.
Nature: Jolly (+Spe, -SpA) or Adamant (+Atk, -SpA)
EVs: 252 Speed, 252 Attack
Moveset:
- Night Slash
- Rock Slide
- Acrobatics
- Toxic Spikes
Other good moves are Aerial Ace (if you want a held item, switch this with Acrobatics), Shadow Sneak, Waterfall, U-Turn, and Power-Up Punch.
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Overall Analysis:
Your team is very offensive. This is common for, say, story teams because it's the easiest way to play. You have good type coverage but not much strategy. If you use Baton Pass Mawile then you'll have something unique and fun to mess with. Otherwise, you're just trying your best to one-shot or two-shot every Pokemon. This team will work for battles with friends and can even have some success in random battles.
Once you've played around with this team some, I'd suggest looking into some more defensive or supportive team members. Walls are used to handle your opponents' sweepers and set-up traps or defenses to make your whole team better. If you want to do double, triple, or rotation battles, then having support Pokemon is a good idea too. They will help power up your Pokemon or weaken your opponents and are valuable in these types of battles.
