I'll do my best. :P
Heliolisk, Mega Ampharos, Rotom (Wash), or Magnezone are the best non-Legendary Electric-Type Pokémon that are available to you in X. I think Heliolisk is good enough. :P
For Heliolisk, if you use Rain Dance, Give it Leftovers. If you don't use Rain Dance, give it a Choice Scarf, and Replace Rain Dance with Focus Blast or Grass Knot. (Focus Bast is better overall, but Grass Knot is more Accurate).
Most of his team are Dominantly physical Pokémon (Charizard looks to be his only Special Pokémon), so your Klefki should have 252 Def, not SpD, and a Bold Nature, if possible. Give it Foul Play, Spikes, Thunder Wave, and Substitute, and have it hold Leftovers. Spikes hurts a lot of his team, and Thunder Wave+Substitute is for annoying Hax. :P
Aegislash will do better with Leftovers, unless if you are able to spam Autotomize 4 times without being KO'd and have Weakness Policy activate, which is highly unlikely. Swords Dance is a good move for offensive Aegislash. Get an Adamant or Impish nature, if possible.
Heracross does better with Close Combat rather than Brick Break. Replace Mega Horn with Facade (it does more neutral damage). Give Heracross Earthquake and Rock Slide/Stone Edge. Give it an Adamant or Jolly Nature, if possible. While I am all for Guts+Flame Orb, if you want to destroy you're Friend's Charizard, you may consider Scarfing Heracross. Heracross may be slower than Charizard, depending on its Nature, IVs, and EVs, so if you want to be very sure you OHKO it, give Heracross a Choice Scarf and use a Rock move against Charizard. :P
Greninja with Torrent may work out for you in a way. Try to get a Hasty/Naive Nature, if possible. Give Greninja a Focus Sash. Teach it Extrasensory and Rock Slide. Use this spread:
252 SpA, 252 Spe, 4 Atk. If your Greninja's Focus Sash isn't broken, it will 2HKO Charizard and most likely take enough damage to activate Torrent, which powers up Scald (still, don't use water type moves in Harsh Sunlight). Ice Beam Should OHKO Garchomp whether it mega Evolves or not.
Well, it probably is a good idea to get a Ferrothorn of your own. I suggest you use something like this as your 6th Pokémon:
Ferrothorn @ Rocky Helmet
Ability: Iron Barbs
EVs: 248 HP / 8 Atk / 252 Def
IVs: 0 Spe
- Gyro Ball / Power Whip
- Stealth Rock / Ingrain
- Leech Seed / Ingrain
Ingrain/Leech Seed is for Healing. If your freind is only using Speed and Attack EVs, not only would his Ferrothorn be really bad, but it will also kill itself through contact damage on your Ferrothorn. Just Remember, you can't Leech Seed his Ferrothorn, so if you don't run ingrain, watch out for taking too much damage. :P
Your Pokémon are pretty good for countering what you friend has, but the biggest problems I see you having in fighting his team is probably Charizard if it mega Evolves, and Crobat if he can Hax flinch. Otherwise, I think you should do fine. :P
Hope I Helped!