Scizor @ Scizorite  
Ability: Light Metal  
EVs: 100 HP / 252 Atk / 84 Def / 72 Spe  
Naughty Nature  
- Bullet Punch  
- Hidden Power [Fire]  
- Aerial Ace  
- Swords Dance

What it beats:

Dwebble
       M-Scizor lives a z-rock wrecker on a critical hit from a lonely dwebble and always KOs with Hidden Power [Fire] to Bullet Punch.
       252+ Atk Dwebble Continental Crush (200 BP) vs. 100 HP / 84 Def Scizor-Mega on a critical hit: 241-285 (78.7 - 93.1%) -- guaranteed 2HKO 
       0 SpA Technician Scizor-Mega Hidden Power Fire vs. 0 HP / 0 SpD Dwebble: 102-120 (42.3 - 49.7%) -- guaranteed 3HKO 
       72+ Atk Technician Scizor-Mega Bullet Punch vs. 0 HP / 4 Def Dwebble: 242-288 (100.4 - 119.5%) -- guaranteed OHKO 

Pinsir-Mega
       M-Scizor lives an adamant Mega-Pinsir Giga Impact and always KOs with Aerial Ace into Bullet Punch. However, be wary of bulk up Mega-Pinsir       who will beat M-Scizor.
       252+ Atk Aerilate Pinsir-Mega Giga Impact vs. 100 HP / 84 Def Scizor-Mega: 258-304 (84.3 - 99.3%) -- guaranteed 2HKO 
       72+ Atk Technician Scizor-Mega Aerial Ace vs. 0 HP / 4 Def Pinsir-Mega: 182-216 (67.1 - 79.7%) -- guaranteed 2HKO 
       72+ Atk Technician Scizor-Mega Bullet Punch vs. 0 HP / 4 Def Pinsir-Mega: 91-108 (33.5 - 39.8%) -- guaranteed 3HKO 

Scizor-Mega
       M-Scizor is able to beat other M-Scizor's with Hidden Power [Fire].
       252+ Atk Technician Scizor-Mega Aerial Ace vs. 100 HP / 84 Def Scizor-Mega: 85-100 (27.7 - 32.6%) -- guaranteed 4HKO 
       0 SpA Technician Scizor-Mega Hidden Power Fire vs. 248 HP / 0 SpD Scizor-Mega: 184-220 (53.6 - 64.1%) -- guaranteed 2HKO

Heracross-Mega
       M-Scizor is able to beat Mhera with Aerial Ace. But again, be wary of Bulk Up Mhera with close combat.
       252+ Atk Heracross-Mega Close Combat vs. 100 HP / 84 Def Scizor-Mega: 198-234 (64.7 - 76.4%) -- guaranteed 2HKO 
       72+ Atk Technician Scizor-Mega Aerial Ace vs. 252 HP / 0 Def Heracross-Mega: 380-448 (104.3 - 123%) -- guaranteed OHKO 

Durant
       M-Scizor is able to beat Durant by living a hit and OHKOing with Hidde Power [Fire]
       252+ Atk Hustle Durant Continental Crush (180 BP) vs. 100 HP / 84 Def Scizor-Mega: 200-236 (65.3 - 77.1%) -- guaranteed 2HKO 
       0 SpA Technician Scizor-Mega Hidden Power Fire vs. 0 HP / 0 SpD Durant: 328-388 (127.6 - 150.9%) -- guaranteed OHKO 

Armaldo
       M-Scizor is able to beat Armaldo by 2 or (very rarely) 3HKOing with Bullet Punch and living 2 Hidden Power [Fire]s from Armaldo.
       126 SpA Armaldo Hidden Power Fire vs. 100 HP / 250 SpD Scizor-Mega: 124-148 (40.5 - 48.3%) -- guaranteed 3HKO 
       72+ Atk Technician Scizor-Mega Bullet Punch vs. 252 HP / 116+ Def Armaldo: 174-206 (49.1 - 58.1%) -- 98.4% chance to 2HKO 

Buzzwole
        (note: at this point I had to change the set. but I don't particularly care to redo all the calcs before this). M-Scizor beats Buzzwole by always 2hkoing with Aerial Ace and living a +1 z-bounce. However, super bulky Buzzwoles may be able to get a win.
       +1 252+ Atk Buzzwole Supersonic Skystrike (160 BP) vs. 100 HP / 84 Def Scizor-Mega: 211-249 (68.9 - 81.3%) -- guaranteed 2HKO 
       252+ Atk Technician Scizor-Mega Aerial Ace vs. +1 252 HP / 0 Def Buzzwole: 244-288 (58.3 - 68.8%) -- guaranteed 2HKO 
       252+ Atk Technician Scizor-Mega Aerial Ace vs. +2 252 HP / 0 Def Buzzwole: 180-216 (43 - 51.6%) -- 6.3% chance to 2HKO 

Shuckle
       SD + Bullet Punch Should always beat Shuckle unless it get's lucky with Acupressure.
       +6 252+ Atk Technician Scizor-Mega Bullet Punch vs. +3 248 HP / 252+ Def Shuckle: 150-176 (61.7 - 72.4%) -- guaranteed 2HKO 

Scolipede
        Aerial Ace into a bullet punch always OHKOs.
        252+ Atk Technician Scizor-Mega Aerial Ace vs. 252 HP / 0 Def Scolipede: 264-312 (81.4 - 96.2%) -- guaranteed 2HKO
        252+ Atk Scolipede Continental Crush (180 BP) vs. 100 HP / 84 Def Scizor-Mega: 126-149 (41.1 - 48.6%) -- guaranteed 3HKO 

Forretress
        Always gets 2HKOed by Hidden Power [Fire]
        0 SpA Technician Scizor-Mega Hidden Power Fire vs. 252 HP / 4 SpD Forretress: 272-324 (76.8 - 91.5%) -- guaranteed 2HKO 
        -1 0 SpA Technician Scizor-Mega Hidden Power Fire vs. 252 HP / 4 SpD Forretress: 180-216 (50.8 - 61%) -- guaranteed 2HKO 
        252+ Atk Choice Band Forretress Revenge vs. 100 HP / 84 Def Scizor-Mega: 118-139 (38.5 - 45.4%) -- guaranteed 3HKO

Araquanid
       252+ Atk Water Bubble Araquanid Hydro Vortex (160 BP) vs. 100 HP / 84 Def Scizor-Mega: 270-318 (88.2 - 103.9%) -- 25% chance to OHKO 
       252+ Atk Technician Scizor-Mega Aerial Ace vs. 252 HP / 0 Def Araquanid: 258-304 (75.8 - 89.4%) -- guaranteed 2HKO 

Scyther
      Scyther can win with 252 modest Hidden Power Fire, but M-Scizor should win otherwise.
       0+ SpA Technician Scyther Hidden Power Fire vs. 100 HP / 0- SpD Scizor-Mega: 200-236 (65.3 - 77.1%) -- guaranteed 2HKO 
       252 Atk Technician Scyther Quick Attack vs. 100 HP / 84 Def Scizor-Mega: 20-24 (6.5 - 7.8%) -- possibly the worst move ever 
       252+ Atk Technician Scizor-Mega Aerial Ace vs. 248 HP / 0 Def Scyther: 288-340 (83.9 - 99.1%) -- guaranteed 2HKO 

Escavalier
       0 SpA Technician Scizor-Mega Hidden Power Fire vs. 252 HP / 8 SpD Escavalier: 176-208 (51.1 - 60.4%) -- guaranteed 2HKO 
       And...Escavalier can't really hit you at all.
       252+ SpA Escavalier Hidden Power Fire vs. 100 HP / 0- SpD Scizor-Mega: 200-236 (65.3 - 77.1%) -- guaranteed 2HKO

Yanmega @ Flyinium Z
      252+ SpA Yanmega Supersonic Skystrike (140 BP) vs. 100 HP / 0- SpD Scizor-Mega: 258-304 (84.3 - 99.3%) -- guaranteed 2HKO 
      252+ Atk Technician Scizor-Mega Aerial Ace vs. 0 HP / 0- Def Yanmega: 304-358 (97.1 - 114.3%) -- 81.3% chance to OHKO 

Beedrill-Mega
      252+ Atk Beedrill-Mega Drill Run vs. 100 HP / 84 Def Scizor-Mega: 75-89 (24.5 - 29%) -- 99.8% chance to 4HKO 
      252+ Atk Technician Scizor-Mega Aerial Ace vs. 0 HP / 0 Def Beedrill-Mega: 486-574 (179.3 - 211.8%) -- guaranteed OHKO 

Ribombee
      252+ Atk Technician Scizor-Mega Bullet Punch vs. 0 HP / 0 Def Ribombee: 362-428 (138.6 - 163.9%) -- guaranteed OHKO       

Ninjask
        252+ Atk Ninjask Supersonic Skystrike (120 BP) vs. 100 HP / 84 Def Scizor-Mega: 118-139 (38.5 - 45.4%) -- guaranteed 3HKO 
        252+ Atk Technician Scizor-Mega Aerial Ace vs. 0 HP / 4 Def Ninjask: 444-524 (168.8 - 199.2%) -- guaranteed OHKO 

Shedinja
        Just Aerial Ace always wins. No calcs needed.

What beats it:

Volcarona
All Volcaronas beat Mega-Scizor by outspeeding and OHKOing with it's fire STAB.

Vikavolt
Vikavolt is able to easily tank two Hidden Power [Fire]s, use charge,  and KO with the electric z-move.
       0 SpA Technician Scizor-Mega Hidden Power Fire vs. 252 HP / 0 SpD Vikavolt: 116-138 (32.4 - 38.5%) -- 98.6% chance to 3HKO 
       0 SpA Technician Scizor-Mega Hidden Power Fire vs. +1 252 HP / 0 SpD Vikavolt: 78-92 (21.7 - 25.6%) -- 1% chance to 4HKO 
       +2 4 SpA Vikavolt Gigavolt Havoc (185 BP) vs. 100 HP / 250 SpD Scizor-Mega: 436-514 (142.4 - 167.9%) -- guaranteed OHKO 
       (+2 to simulate charge)

Galvantula
Does beat Scizor-Mega. You can ev for it...but I think the opportunity cost is too high (100%ing Forretress)
       252+ SpA Galvantula Gigavolt Havoc (185 BP) vs. 100 HP / 0- SpD Scizor-Mega: 303-357 (99 - 116.6%) -- 93.8% chance to OHKO 

Accelgor
It does beat Scizor-Mega.
       252+ Atk Technician Scizor-Mega Bullet Punch vs. 0 HP / 4 Def Accelgor: 241-285 (80 - 94.6%) -- guaranteed 2HKO
       252 SpA Choice Specs Accelgor Hidden Power Fire vs. 100 HP / 0- SpD Scizor-Mega: 364-432 (118.9 - 141.1%) -- guaranteed OHKO 

Yanmega @ Bugnium Z
       252 SpA Tinted Lens Yanmega Savage Spin-Out (175 BP) vs. 100 HP / 0- SpD Scizor-Mega: 294-346 (96 - 113%) -- 75% chance to OHKO 

50/50s:
Golisopod
       252+ Atk Golisopod Hydro Vortex (160 BP) vs. 100 HP / 84 Def Scizor-Mega: 195-231 (63.7 - 75.4%) -- guaranteed 2HKO 
       252+ Atk Choice Band Golisopod Aqua Jet vs. 100 HP / 84 Def Scizor-Mega: 75-88 (24.5 - 28.7%) -- 98.9% chance to 4HKO 
       252+ Atk Technician Scizor-Mega Aerial Ace vs. 252 HP / 0 Def Golisopod: 180-212 (50.8 - 59.8%) -- guaranteed 2HKO 
Unsure:

Yanma
         Hypnosis + Substitute + Air Slash + Roost could wear Scizor-M down over time if it is lacking roost. But it has to manage to get enough sleep turns to roost back to full AND be able to substitute.

Pheromosa
         Specs Hidden Power [Fire] Phero OHKOs, but I don't know what Pheros run at all.

Vivillon 
         Sleep powder sub should theoretically win...given enough sleep turns. But, it needs at least two sleep turns...and I don't know if that is the standard Vivillon set.

Venomoth
         Sleep powder can cheese out wins here. Again, i'm not sure at all what they run.

Good teammates:

Volcarona @ Choice Scarf  
Ability: Flame Body  
EVs: 252 SpA / 4 SpD / 252 Spe  
Modest Nature  
IVs: 0 Atk  
- Flamethrower  
- Overheat  
- Hurricane  
- Hidden Power [Rock]
Choice Scarf Volcarona is capable of beating all the things that I was unsure about as well as Yanmega, Vikavolt, Accelgor, and Galantula. As well as 50/50 other Scarf Volcaronas.