So you want it six ivs...?
If yes, then the best way would be to breed a six Iv pokemon to a another six iv(or five, but six is more reliable) and then you have a 1/32 chance (the best chance you could ever get) to get a six Iv Garchomp.
That with the random nature problem (1/25) and the fact that whether it gets the HA or not (1/2) would be around 1/410 chance, and that's hard. Better throw in items like ever stone, you'll still have a 1/50 chance though