Yaay!! Double Battles!!
First, let's look at Mega Garchomp's base stats.
HP 108 / Atk 170 / Def 115 / SAtk 120 / SDef 95 / Spd 92 (Source)
Most people train Garchomp's Spd and Atk, because although the SAtk is high, Atk is way higher (high enough to rank next to some of the most powerful physical attackers in Ubers). So let's run a physical Garchomp set to maximize his offensive potential.
Also, we'll visit his Nature and make it boost its Spd and lower its SAtk, because we won't be using that.
Garchomp @ Garchompite
Ability: Rough Skin
EVs: 252 Atk / 252 Spd / 4 HP
- Dragon Claw
- Swords Dance
- Rock Slide/Iron Head
With this set, you double his Atk with Swords Dance, boosting his ridiculously high Atk to insane levels. Dragon Claw is your Dragon-type STAB, and Earthquake is your Ground-type STAB. Rock Slide makes good coverage next to Earthquake and hits both of the opponent's Pokemon, or you can run Iron Head to cover Fairies. The only Fairy you would have to worry about outspeeding Garchomp would be scarfed Togekiss, making Iron Head viable. However, I would recommend Rock Slide, as I would probably switch if I saw a Togekiss coming at Garchomp. Empoleon covers your Fairy weakness, so you would have a good option for a switch-in.
Next, let's visit Togekiss's set.
Isn't Togekiss just adorable? Yeah, it is.
I think a really good set for you to run next to Mega Garchomp would be a paraflinch abuse set. Let's make it look like this:
Togekiss @ Leftovers
Ability: Serene Grace
EVs: 252 SAtk / 252 HP/ 4 Spd
- Thunder Wave
- Air Slash
- Aura Sphere
Here, you totally abuse the ability Serene Grace as it boosts the likelihood of added effects appearing (e.g., flinching when hit with Air Slash). Before you start abusing flinching, you paralyze the opponent, giving you an edge on Spd and further decreasing the likelihood that the opponent will be able to move at all. With a Flying-Fighting move core, you will be able to hit all Pokemon with at least neutral damage, meaning nothing will resist Togekiss's attacks, giving you a further edge on a field where more than one Pokemon is there to be reckoned with.
Garchomp and Togekiss only share one weakness-- ICE. Although you will probably not see very many Ice-types at all aside from Mamoswine, Ice Beam is a common move, meaning that this could be a problem. Garchomp is actually 4x weak to Ice. Despite this hindrance, I think these two make a good core. Just play smart and know when to anticipate an Ice-type attack.
Empoleon has both bulky and offensive potential. I say we invest in both!
Empoleon @ Leftovers
Ability: Defiant (Torrent is fine, if you can't access the Hidden Ability)
EVs: 252 HP / 252 SAtk / 4 SDef
- Mud Sport/Protect
- Aqua Ring
- Flash Cannon
This is your new bulky Empoleon. Leftovers with Aqua Ring means it's restoring 12% of its HP each turn. Scald has a chance to burn the opponent, halving their Atk, making your bulk just that much more effective. Flash Cannon is STAB and mostly for Fairies, because you need coverage for Garchomp. Also, it completely absorbs Poison-type moves and resists Rock-types for Togekiss, making Empoleon a valuable member of your team. It even resists Ice, helping both Garchomp and Togekiss further. The problem is that Empoleon shares an Electric weakness with Togekiss. To fix this problem, we'll run Mud Sport to halve Electric-type moves' power, but you could also run Protect to heal for a turn, because Garchomp's Ground typing means it absorbs Electric moves. Empoleon is a valuable member of your team because it could work next to either Garchomp or Togekiss if one of them were to get KO'd before the other. Togekiss also absorbs Fighting-type attacks for Empoleon. Together, these three Pokemon resist everything at least once. This sort of balance embodies what double battles are all about!
Next, you've got Typhlosion as Empoleon's partner.
Typlosion actually has a lot of weaknesses in common with its partners, meaning that it is not as useful to your team as Empoleon, for example. I would consider switching Typhlosion for a Pokemon that can support its teammates better. Something else that Garchomp can Earthquake with could be nice, and the Pokemon I'm thinking of is also immune to Fighting-types...
Gengar @ Black Slude
EVs: 252 SAtk / 252 Spd / 4 HP
- Shadow Ball
- Sludge Bomb
This is a subbing-disable Gengar set. You can Substitute so it doesn't hit you and then disable a move that would be supereffective against the other teammates, like Ice Beam. Using this set requires prediction skill, but it would suit your team well! Also, Gengar's Levitate means Garchomp can quake away without having to worry about his partner.
Let me know if I can help you out in any way! I think I've still got Gastly left over, if you want one with really good IVs. You know to talk to me if you want me to breed you anything. Good luck and have fun, Zach!