Heracross, the Singlehorn Pokémon, is an adept sweeper, and its Mega Evolution is one of the most terrifying wallbreakers in the game. Seriously, Mega Heracross has a Base Attack stat that is greater than Attack Deoxys (marginally mitigated by the fact that it does not have the capability to hold an item). Of course all of that power means nothing if you can't land a hit. Heracross is reliant on a Choice Scarf to patch up its speed issues and Mega Heracross is in an unfortunate dead zone of speed, being regularly outsped both in and out of Trick Room. However, the increased bulk can be a life saver.
Beetle of Atlas
Heracross
Ability: Guts/Moxie (Skill Link)
Item: Heracrossite
-Bulk Up
-Pin Missile
-Rock Blast
-Arm Thrust/Bullet Seed
Nature: Adamant (+ATK, -SATK)
EVs: 252 HP/4 ATK/252 SDEF
Bulk Up Mega Heracross can be a bit of a tank, provided you keep it away from any flying attacks. At Max HP and SDEF, Mega Heracross can take some of the biggest special hits in the game:
252 SpA Chandelure Fire Blast vs. 252 HP / 252 SpD Heracross-Mega: 300-354 (82.4 - 97.2%) -- guaranteed 2HKO
252 SpA Alakazam-Mega Psychic vs. 252 HP / 252 SpD Heracross-Mega: 282-332 (77.4 - 91.2%) -- guaranteed 2HKO
And after 1 Bulk Up, can do the same on the Physical side as well:
252 Atk Pure Power Medicham-Mega Zen Headbutt vs. +1 252 HP / 0 Def Heracross-Mega: 258-306 (70.8 - 84%) -- guaranteed 2HKO
252 Atk Tough Claws Charizard-Mega-X Flare Blitz vs. +1 252 HP / 0 Def Heracross-Mega: 302-356 (82.9 - 97.8%) -- guaranteed 2HKO
While still being solidly 2HKOd (or OHKOd after residual damage), the fact that Mega Heracross can take these hits is a testament to how bulky it is and can surprise a faster sweeper looking for a revenge kill. The lack of recovery hurts... a lot, but Mega Heracross is to remove the defensive lynchpin from the opposing team, opening the way for the endgame sweep. As for the attacking moves, Pin Missile is for STAB, Rock Blast is for coverage and catch predicted Fire and Flying switch ins, Arm Thrust is preferred here as secondary STAB as Close Combat defeats the point of Bulk Up/the defensive investment, and Bullet Seed is a decent alternative option.
Speedy Beedle
Heracross
Ability: Moxie
Item: Choice Scarf
-Megahorn
-Close Combat
-Stone Edge
-Knock Off/Earthquake
Nature: Jolly (+SPE, -SATK)
EVs: 4 HP/252 ATK/252 SPE
Scarfed Heracross + Moxie = Terrifying late game cleaner/revenge killer. Once the opposing team has been weakened sufficiently, normal Heracross can come in for the kill. With a Choice Scarf and speed boosting nature, Heracross outspeeds all of UU (Jolly + Scarf = 445 speed vs Mega Aerodactyl's 438), unless the opponent is packing a speed boost/Choice Scarf of their own. Moxie allows Heracross to snowball, becoming stronger with each KO. Megahorn and Close Combat are for STAB, Stone Edge is to catch flying types who resist both STABs, Knock Off offers some early game utility, while Earthquake catches Infernape, Mega Manectric, and Nidoking hard.
Standard Sweeper EVs.
Other Options
Guts + Flame Orb + Swords Dance = BURNING HORN! Which is how normal Heracross is a terrifying wallbreaker.
Pursuit -Heracross is very scary and can cause switches
Feint (VGC Only) -Protect, Wide Guard and Quick Guard are prominent moves in VGC, so removing them can lead to turmoil with your opponent's strategy.
Smack Down -Hilarious if done on a Skarmory or Celesteela, not much worth otherwise.