Go to the Stone Emporium in Lumiose city. there's a guy in there that sells the mega stones for the kanto starters after you beat the elite 4. the prices are ridiculous, but it would allow you to buy an extra charizard stone of the same version you're in.
I am aware of the guy that sells the mega stones to you (you can actually get them as soon as you can find the store in vernal avenue, and the price can be reduced by improving your "style"), but I went with squirtle as my kanto starter, so will he sell me a venasaurite and TWO mega charizard y stones, or just 1? that's want I wanna know.
mainly it's just not fast enough to outspeed things that can take advantage of its many weaknesses, including its two 4x weaknesses: fighting and steel. This was a problem I ran into alot while trying to use him. doesnt matter though, any pokemon is viable in story mode if you train it high enough