Garchomp @ Life Orb
- Dragon Claw
- Fire Blast
- Stone Edge
I don't know what metagame you're using this in, or if it is in-game or competitive, which is pretty important information as that would drastically change my choices (for example, an all-out attacking Garchomp like this one would be unviable in SM OU) but for the sake of answering I'll optimize the set with the options you gave me.
Starting with the Fire moves, I chose Fire Blast because it is more effective than the other options. Its purpose is to hit Pokemon that Garchomp's STAB moves can't, and these Pokemon, such as Skarmory and Tapu Bulu, are generally hit harder by Special moves, which eliminated Fire Fang as a possibility. Flamethrower is simply unviable on its own due to its low base power, leaving Fire Blast as my choice. Because of this, I added a Life Orb to the set even though you did not ask for an item, as it is needed to boost Fire Blast's power to an acceptable level. In addition, I chose Naive as it boosts Garchomp's Attack while lowering its Special Defense, which isn't too bad because it is already mediocre. Lastly, I chose Stone Edge over Rock Slide because, once again, Garchomp appreciates the extra power.