Basically, bring Mega Venu against teams that Char-Y will probably be bad against (Rain with Mega Swampy, and Sand with TTar+Excadrill for example) if you don't feel confident duking out a weather war like the wonderful days of Gen 5 all over again, or if your foe lacks strong Flying / Psychic coverage (Metagross, Mence, Talonflame, don't worry about Cress too much) they probably can't break Mega Venusaur. Japan Sand archetypes for example (TTar + Excadrill + Mega Mence + Amoonguss) get smashed by Mega Venusaur once Mence is down, and Swampy excels against TTar/Excadrill/Mega Mence. Char-Y, gets crushed by TTar and Mega Mence, so it's not the best thing to bring.
If they have more vulnerability to Char-Y, like Mega Gardevoir + Amoonguss, Lando-T + Rotom-W, Mega Metagross + Hydreigon + Sylveon, etc. just go right ahead and spam Heat Wave with absolutely no mercy, pity, or remorse, and if you see a single target you dislike, like Khan or Landog, just nuke it.
It's hard to get used to the double Mega thing at first, but you'll get the hang of it. I've been using the core a long time, so I'm used to all the double mega shit and Pledge mechanics, I understand it's not user-friendly when first trying it. But, I can answer any questions on it :P
edit: You would not bring Charizard at all when you pick Mega Venusaur, non-Mega Charizard is kinda bad. I almost always bring Swampert in just about every match regardless of which mega though.
