Mega Heracross is a spectacular, if somewhat strange, Mega Evolution. Its incredible Attack is second only to Mega Mewtwo X, so yes, it actually can hit harder than a Primal Groudon. In addition, it gets some nice boosts to its defenses, allowing it to take a few neutral hits before going down. It does, however, lose a much-needed 10 base speed from its pre-Mega version; even with a Jolly nature, Mega Heracross struggles to keep pace with the speedsters of OU/BSS. Unfortunately, Mega Heracross just falls short of being great in OU/BSS due to its speed and the fact that there are too many big threats to it; that 4x weakness to Flying makes it ready bait for any Talonflame or Mega Salamence. Still, Mega Heracross can perform a fairly unique role on a team and is very good at it.
Mega Heracross only really has one set with variations, but it works wonders, breaking any Sash or Sturdy user imaginable. Here's one I've used many times:
EVs/Nature: 252 Atk / 4 Def / 252 Spe, Jolly Nature
Ability: Skill Link (Moxie on pre-Mega)
- Close Combat / Arm Thrust
- Pin Missile
- Rock Blast
- Swords Dance / Bullet Seed
This is a set based around speed and raw attack power, an odd set but it still outpaces several rather important threats and still has that amazing damage output. Mega Heracross is a great wallbreaker, and this set brings that same capability to bear against a few faster threats, such as Breloom, which outpaces bulky sets and can use Spore to shut them down. Close Combat is the primary STAB option in the first slot, although Arm Thrust can be used to substitute power for a lack of recoil and more Sash/Sturdy breaking. However, going with Arm Thrust costs you most opportunities to use Moxie prior to Mega Evolving, so you'll use CC most of the time. Pin Missile is your other STAB and hits plenty hard, and Rock Blast gets excellent coverage. This set isn't really bulky enough for Substitute, so in the last slot we have Swords Dance to make Mega Heracross's attack all kinds of ridiculous or Bullet Seed for additional coverage, very nice for those bulky Water-types too.
Countering Mega Heracross is a difficult task, but it can be done. The most obvious answer is Flying types, since Heracross has a nasty 4x weakness to them, and they resist both of its STABs. Quick revenge killers have no trouble with Heracross after Close Combat recoil. Fairies don't usually mind it too muck either, although they may be vulnerable to its coverage; Azumarill hates taking a Bullet Seed. Fire types may fare well, but watch out for a Rock Blast on the switch, or a Close Combat/Arm Thrust if you throw in Heatran. Just remember that it's easier to revenge kill Mega Heracross than it is to switch into it.
Not sure how good Heracross does as a pre-Mega, or if it's any good in Doubles. Hopefully I'll learn something this week.
The risk I took was calculated, but man, am I bad at math.