Let's look at its stats. It has a gargantuan attack stat of 185. While its defence stats are not that bad with 80 HP, 115 Def and 105 Sp.Def, its speed is mediocre, with 273 Spe at max speed nature. Also, non-mega Heracross was bit faster at 85. You can see its viability here, it is ranked at B-. In my opinion, I had never bother with having 252 spe on it. I would like to run it on Trick Room team only, since its weakness are easily targeted, with the most viable Pokemon Cinderace, Clefable, Gliscor, Heatran, Toxapex ranked at S aren't totally bothered with whatever M-Heracross carries. I know that Cinderace gets checked with Rock Blast, still it will OHKO you with Pyro Ball or dynamaxed Bounce becoming Max Airstream. If you need Pokemon with high attack, there are still much viable options like M-Mawile, M-Lopunny and M-Scizor, M-Medicham, etc. Except Mawile, the other options are still faster than M-Heracross, and prove more viable. Plus, they do check more common Pokemon compared to M-Heracross.
About the moveset of yours.... It depends on what kind of team you run. I am telling one about a trick room one:
Heracross-Mega @ Heracronite
Ability: Skill Link
EVs: 248 HP / 252 Atk / 8 SpD
Brave Nature
- Close Combat
- Pin Missile
- Bullet Seed
- Rock Blast
Hope this helps!
