Mega Heracross has a base speed of 70, while Mega Garchomp has a base speed of 92. Chances are, no matter what you do, Mega Heracross is going to be outsped by Mega Garchomp. However there are quite a few megas who do very well 1v1 vs Mega Garchomp, even when limited to XY megas.
Mega Kangaskhan
![enter image description here][1]
Although it holds no type advantage against Garchomp, Mega Kangaskhan is ridiculously powerful. It's parental bond ability means it's attacks all hit twice and have 1.5 times normal power, and it's base speed of 100 allows it outspeed Mega Garchomp when properly trained. Fake Out can provide free damage, Power-up-Punch can allow you to boost or you can opt to hit extremely hard right off the bat with Return or Double Edge. Note that Double Edge is a breeding move, but Aggron is a potential parent that can pass down Double Edge and you seem to have one ready for that. Mega Kangaskhan is just a constantly reliable Pokemon to have. Just watch out for normal Garchomp's rough skin ability, because it could cause Kangaskhan to take 25% damage every time it attacks.
Mega Gardevoir
![enter image description here][2]
If there's one thing that's guaranteed to OHKO his Garchomp, it'd be Mega Gardevoir. At base 100 it also outspeeds Mega Garchomp, but be warned, before they Mega Evolve, Garchomp outspeeds it. It's STAB Moonblast (or Hyper voice if you have access to that) will leave a dent in nearly anything. Be warned that Mega Gardevoir has very weak defense and that an Earthquake from Mega Garchomp could potentially KO it.
Mega Blastoise
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Although Mega Blastoise doesn't outspeed Garchomp or it's mega, it has the bulk to avoid 2HKO from a good portion of its moves and Ice Beam which will likely OHKO it. Dragon Pulse may also be used as it is boosted by Mega Launcher but it remains weaker than a 4x effective Ice Beam and it is a breeding move, making it typically not worth the time.
Mega Medicham
![enter image description here][4]
What Medicham lacks in bulk it makes up for in power. It's Pure Power ability makes it hit like a truck, and at base 100 it may fail to outspeed regular garchomp but it outspeed the mega. Note that on the first turn it's base speed will be 80 and it will likely go last unless you invest fully in speed and this garchomp is a bulkier variant. Like Kangaskhan, Medicham can make up for this initial speed issue by using Fake Out on the first turn, and while it may not have Kangaskhan's bulk Medicham can learn Ice Punch via move relearner, gaining an almost guaranteed ko on Garchomp and a definitely guaranteed one after Fake Out. Note that like Gardevoir Medicham is frial and may fall to an Earthquake.
Mega Gyarados
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Mega Gyarados really combines what makes the previous four megas a solid choice into one package. Like Medicham and Blastoise is has super effective Ice Coverage in the form of ice fang, although ice fang won't OHKO of the bat. Like Gardvoir it has a really solid STAB and coverage combination, and like both Blastioise and Kangaskhan it has a considerable amount of bulk. What sets Mega Gyarados apart is that it can set up Dragon Dance in Garchomp's face, outspeed it the next turn and OHKO with Ice fang and then sweep the rest of his team. It's intimidate ability weakens Garchomp's powerful physical attacks, and Mega Gyarados has such good special bulk that it'll shrug off even a draco meteor with ease. It's easiness to catch, breed and train and lack of necessary egg moves also make it an easily accessible choice.
What I failed to mention is that there's plenty off non-mega Pokemon that kick Garchomp but. Protean Greninja can easily outspeed it and OHKO it with Ice Beam. Azumarill and Clefable both have great bulk and can one or two hit ko it with fairy moves. Gliscor can toxic stall it out, Togekiss is immune to both it's STABs, the list goes on, but if you're heartset on a mega clash, any of the five above are good choices. Abomasnow, and if you have ORAS, Altaria and Salamence, are solid mega choices as well.
Hope this helps.
[1]: http://img.pokemondb.net/artwork/kangaskhan-mega.jpg
[2]: http://oyster.ignimgs.com/mediawiki/apis.ign.com/pokemon-x-y-version/thumb/9/98/Mega_gardevoir.jpg/228px-Mega_gardevoir.jpg
[3]: http://img.pokemondb.net/artwork/blastoise-mega.jpg
[4]: http://pokecharms.com/data/attachment-files/2014/03/62908_308Medicham_Mega_Dream.png
[5]: http://img1.wikia.nocookie.net/__cb20140605022945/vsbattles/images/7/71/Mega_Gyarados.png