Ive been using Msaur alot (the only Mega i use) and its realy reliable with this set:
Venusaur @ Venusaurite
Calm Nature
Leech seed
Toxic
Giga drain
Sludge bomb
I use Leech seed over synthesis, because it also help teamate if you ever need to switch and depending of the enemie Msaur can set a leech seed and still live with is bulk. Poisonheal-Gliscor and Leftover-Rotom love this extra recovery. And sometime its easy to predict the switch to talonflame from the foe, with this, talonflame fall more early. Also, in some situation, the continuous recovery from leech seed is better than synthesis, ifMsaur ever get paralysed/frozen/confuse/or is asleep. If one leech seed is set its quite a relief to still get some HP for the turn you might miss.
Toxic is there to cripple the pkmn who are more bulky, Since i have a problem with accurcy...i dont run Sleep Powder. Giga drain is there for more recovery.
I dont have HP-fire but with my Rotom-H this is not a problem. So Sludgebomb is the move to have.
What EVs are you running with that? I like the look of it.