Aight, AV means Assault Vest btw, since Metagross is used as a Special Wall in this case! I'll write a short ddescription here:
Metagross and Garchomp are a perfect balanced core, why? Both of them have an amazing attack and pretty good defenses. Metagross, equipped with an Assault Vest and combined with the right EV spread laughs at most of the special hits. It can also be used as a Heatran counter if you run out of options, EQ + 196 EVs invested in Atk will assure a freaking ton of damage that may (and probably will result in a OHKO). Metagross can even survive to one (and if you're lucky 2) of Heatran Lava Plumes, and we all know how hard these hit.
Garchomp is a good partner mainly for his ability: Rough Skin. Combined with Rocky Helmet it literally punishes a lot of physical attackers out there and can live longer enough to set up Stealth Rocks and/or land a powerful hit on the enemy.
The sinergy between them is not so stellar, but if paired with the right 3rd mon they can hurt. A lot. Metagross checks dangerous threats such as Choice Band Azumarill and Latios/Latias that could be annoying for Garchomp. This one covers Meta on threats like Focus Sash Bisharp and annoying Physical attackers that could put Metagross in trouble. I strongly recommend to pair them with Rotom-W to have the best synergy.
About the sets
- Metagross: EQ is for coverage if Chomp goes down somehow and to hit that freaking Heatran. Bullet Punch gives priority and is STAB, as well as Meteor Mash (wich has a chance to raise Metagross Atk) and Zen Headbutt.
- Garchomp: Stealth Rocks doesn't need explanations, as it just sets hazards on the field. Dragon Tail blocks any form of setup (aside Calm Mind Clefable, but for it there is Metagross) and forms a good combo with SR if the player is lucky with the switches as the rocks will continue hurting the new mons on the field. Fire Blast deals with Ferrothorn, a pokémon quite difficult to handle for Metagross. Earthquake is a powerful STAB that hits hard even without EVs.
That ended up longer than expected Please let me know if i have to modify something or be more precise!