I have a Garchomp in Pearl. If this is the game you play, you should go to Route 210 and visit Grandma Wilma. If your Garchomp trusts you, she will teach it Draco Meteor.
Draco Meteor - Power of 140 and an accuracy of 90. PP is only 5 but that can certainly be raised quite a bit.
Sand Tomb - Power is 35, accuracy is 85, PP is 15. It lasts up to five turns. I use it because it pairs well with Dig.
Dig - Power is 80, accuracy is 100, PP is 10. It is a 2 turn move so it's a great dodge, and a good finisher move after the opponent has taken some damage from Sand Tomb. I do this also to avoid being overly reliant on Draco Meteor since the PP is so low.
Flamethrower/Fire Blast - Either is fine for this moveset. I did this mainly for a fire advantage. I went with Flamethrower because it has more PP, and 100% accuracy. Fire Blast has 120 power, but only 85% accuracy.
If you can't get Draco Meteor, you can always teach it Dragon Rush instead, which it learns by leveling up. Only problem with Dragon Rush is the accuracy is 75%.