I don't know if this strategy was done before but I will tell one of the easiest ways to beat Garchomp.
First go to to Onyx Ward and then head to the house between the game corner and move tutor's house.In the upper left corner of the house will be a woman who will give you a seel egg.Once the egg hatches it will have perish song.Then head to North Obsidia Ward and then to the cave.If you check one of the rocks you will find a Focus Sash.The once you battle Solaris sent seel first and use perish song and just wait three turns by switching pokemon till it faints.And that's how i beat him.I hope this helps those who want to progress without losing to him.
