could the M charizard become viable option for competitive play with the help of kiawe?
this card could potentially do 300 damage for 4 fire energy and victini (GUR) could help with the coin flips.
No. You don't have to do 300 damage to anything in the format. The biggest number Fire decks need to shoot for is 230, which is easily and consistently attainable with the current Volcanion builds. Volcanion also doesn't require coin flips (or Victini) to do big damage.
I don't mean to say that Mega Charizard decks can't work, but it's completely outclassed by Volcanion as a competitive deck. Take it to league and leave it there.
could the M charizard become viable option for competitive play with the help of kiawe?
this card could potentially do 300 damage for 4 fire energy and victini (GUR) could help with the coin flips.