You just need a plan of attack. For instance:
You say you would like to use Garchomp. Now personally, I really like the idea of a mixed Mega Garchomp. It has the potential to be an amazing wall breaker.
Garchomp @ Garchompite
Naughty
224 Atk / 32 SpA / 252 Spe
Sand Force
Draco Meteor
Earthquake
Fire Blast
Iron Head
Now, for this set to be most effective, you want to play it under sand. That means you'll want one or both of Hippowdon / Tyranitar. Assuming you're still playing doubles, this makes it arguably easier to keep sandstorm up while maintaining a physical presence for Garchomp.
Now, let's say we want to use Tyranitar. We can't use MegaTar, so that leaves us with a few other options. I'm going to propose we use Hippowdon as our SR user, but we'll come to that later.
Tyranitar @ Assault Vest
Sassy
252 HP / 4 Def / 252 SpD
Sandstream
Crunch / Earthquake
Pursuit
Ice Beam
Rock Slide
Can act as a hard-hitting special sponge, as well as the necessary sandstream starter. Also halts Talonflame completely cold, and can outplay Togekiss.Next, we want something that can potentially help remove some of Garchomp's threats, like Magnezone. It can trap and remove opposing steels such as Skarm / Klefki / Scizor, while also resisting fairies such as Togekiss who make Garchomp's life difficult. Perhaps a Specs set w/ Thunderbolt / HP Fire / Flash Cannon / Volt Switch; or Life Orb with Magnet Rise so you're not forcing Chomp etc. to forgo using EQ when you're on the field.
Then Hippowdon (if you feel another Sandstream user is necessary - which I do, especially because when playing doubles your opponent will know you have to use Chomp + T-Tar if it's your only user) can provide the physical bulk + SR support, as well as your own T-Tar check.
The last two slots I'll leave to you; try to cover common threats or potential weaknesses (as such, this team struggles with Greninja - or any pokemon with a genuinely powerful Surf. Your own bulky water such as Slowbro (regenerator negates sandstream damage) or Jellicent may therefore be an idea, or a water absorber. Thinking outside the box - a Water Absorb Mantine can provide Tailwind support and is immune to EQ thanks to its secondary flying typing. A Rest (Aqua Ring) / Tailwind / Scald / Helping Hand set -or something similar- perhaps?)
Anyway you get the idea. This is all very basic and I didn't put much forward thinking into it, but if you sit down with an idea and a plan, alongside an understanding of common threats, eventually with tweaking you should be able to find a team that can handle MOST situations. (You'll never get a perfect team, obviously)
Hopefully I've at least helped somewhat. Good luck!
