Is one of those Pokemon that can be a major threat when used correctly under the right conditions. It has a beastly 170 base attack which, when partnered with it's ability Sand Force, can take down almost anything it gets a hit on providing sandstorm is up. Sand Force boosts the power of rock, ground, and steel moves when in the sand.
Garchomp also gets access to Hone Claws, a move that boosts attack and accuracy by one stage each. This allows it to take advantage of moves like Iron Tail, a powerful move with normally low accuracy. After one Hone Claws boost, the moves will not only be more powerful, they will also have accuracy just as good as lesser moves, such as dragon claw.
Great, Garchomp is a hard-hitter that can take down nearly all threats in VGC, so why is he not normally used? The answer to that lies in his speed. Mega Garchomp has a not-so-beastly 92 base speed. This is best remedied by Tailwind, a move that doubles the speed of your team's Pokemon for four turns. A good user of this is Togekiss, who also gets access to Follow Me. Follow Me directs all single target moves to the user, and has high priority. With support from Togekiss, Garchomp can get his hone claws off, and also get a needed speed boost.
The finished Garchomp is a powerhouse that can only be easily dealt with before he gets set up.
+1 252+ Atk Mega Garchomp Iron Tail vs. 248 HP / 192 Def Togekiss: 198-234 (103.6 - 122.5%) -- guaranteed OHKO - This kills even without sand!
+1 252+ Atk Mega Garchomp Earthquake vs. 252 HP / 0 Def Aegislash-Shield: 182-216 (108.9 - 129.3%) -- guaranteed OHKO - So does this!
+1 252+ Atk Mega Garchomp Iron Tail vs. 244 HP / 0 Def Sylveon: 324-382 (161.1 - 190%) -- guaranteed OHKO - Iron Tail destroys sylveon!
Take that, VGC!
Mega Garchomp can be an extremely good team member, and when partnered with Togekiss, he can get the boosts he needs to become a major threat to anything he meets. You can even add something with the ability Sand Stream or the move Sandstorm to your team to make Garchomp's attacks do even more.
As long as you watch out for priority attacks and ice attacks, this Garchomp will do you well. You may also want to have a Pokemon on your team to counter starting threats against Garchomp. It is to be noted that Garchomp ideally will use Earthquake, so everything on your team should have Protect.