I just one shot Cynthia's Garchomp

and honestly I am pissed. This pokemon haunted my dreams as a child, I trained day and night, no sleep, with only the will to beat Cynthia. I cannot believe I just beat her Garchomp first time. It's stupid. Wasn't her Garchomp stronger? Did I just use the right pokemon and plan correctly? I don't know and I am so disappointed. Honestly, this whole remake is a disappointment. Any thoughts?