There are three kinds of Garchomp:
1. Garchomp Fail is the standard, average joe Garchomp that is not properly EV trained and is average. Easily defeated by Greninja or any fairy / ice type. Also, Noivern makes short work out of it.
2. Garchomp Scary is the properly EV trained, good Garchomp with maxed out speed and attack. Essentially you need to field someone faster than it with ice beam or a great fairy / dragon attack. Noivern is a good choice. One Draco Meteor and no more problem. Mewtwo, Greninja, Slyveon, etc. all work too. Not sure about Gardevoir as it's not really fast. I've even lost my Garchomp to an ice beam from Aggron of all things. Ice beam is super useful against him simply because he has a 4x weakness to it as compared to 2x of the others. Not to mention almost anyone can use ice beam.
3. Garchomp Impossible is the same as Garchomp Scary except it has choice scarf. So it outspeeds everything. Also, considering the type of player who would go through the trouble to make one of these, it's probably also going to be assisted in major ways such as with Stealth Rock / Spikes / various other support skills. Your only hope is a really powerful fairy type or choice scarf Noivern. However, you can also do funky things such as using thunder wave and will-o-wisp to cripple it provided you find a way to get it off. Generally it's acceptable to suicide a pokemon to do this as no one will have more than one such ultra sweeper.
Generally if someone brings out Garchomp in a 3v3, they've already lost. That 4x weakness to ice beam is too big. Everyone has ice beam.
----
I'm still pretty bad at random battles but I think if you want to have a good time in them, you need to have answers to the following pokemon:
1. Mewtwo (and the other two legendaries)
2. Garchomp (Mega Garchomp is weaker though, don't worry about him)
3. Blaziken
4. Greninja
5. Mega Charizard Y
6. Mega Lucario
7. Aegislash
8. Noivern
9. Snorlax
10. Jolteon