I have two set ideas for Garchomp

Double team
Dig / Earthquake
Dragon claw / Crunch

Nature-Jolly Item-Leftovers
Ability-Sand Stream

Sandstorm and Double team for evasion, plus Sandstorm which is the main reason I put Dig in as an option instead of Earthquake, depends if you want damage from Sandstorm and waste an opponents PP (potential boost) or do more damage in ONE turn. Definently Dragon claw w/ dig and Crunch is optional w/ earthquake if your worried about ghosts or psychics.

Power Chomp

Dragon claw
Fire Fang
Iron Head / Crunch

Nature-Adamant Item-Life orb/Choice band
Ability-Sand Stream

Earthquake and Dragon claw are your stab moves. Fire Fang is for Steels that can take Earthquake and Ice types. Iron head takes out Aerodactyl and Crunch takes out Gengar.

Now time for Double and Triple battles!
Double battle, Evasion set
I would suggest Teaching Garchomp dig w/ a Pokemon that knows surf.

Double Battle, Power Chomp
I would suggest a special hitter like Gengar or Togekiss.

Triple battle, Evasion set
All the other pokemon w/ you Power Hitters that once again know surf

Triple Battle, Power Chomp
Put Garchomp in the middle with pokemon immune to earthquake like Gengar and Skarmory.

and thats the end from me for Garchomp.