To bad that Garchomp's mega form gets a small decrease in its speed because some Fairy, Dragon and Ice types can have a big advantge on it even though it has high attack and yeah anyway Garchomp is a great physical attacker and drgaon/ground type for your team, poor electric types don't even have a advantage because of Garchomp's typeing unless it knows hidden power ice and the you are screwed. In this set it'l make it fast and very offensive as if it already has the perfect IVs for attack and speed or as if it had both a adamant and a jolly nature!
Item: Rock Helmet or Choice Scarf
Ability: Rough Skin
EVs: 252 Attack, 252 Speed and 4 HP
-Stone Edge (if you are going with the Choice Scarf) or Swords Dance (If you are going with the Rocky Helmet)
With a Jolly nature, Garchomp will be swift than usual. With Rough Skin paired up with a Rocky helmet would do serious damage to the opponent if it has a contacting move or you can Scarf it with a Choice Scarf and make its great physical moves monsterous. Dragon Claw is for STAB, great power and perfect accuracy, Earthquake is also a STAB move and has the perfect accuracy and hits alot of your opponents and yes your allys in double or triple battles but still great move. Iron Head is for coverage agenst those big mean sneaky ice types, and those pesky fairy types and it has 30% chance of causeing a flinch. Swords Dance raises attack by 2 stages or you can go with Stone Edge for greater power and a small chance of causeing critical hits.)
READ THIS: just so you know Iron Head is a egg move so you may have to do some mateing spree to your Garchomps.