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Serperior @ Leftovers
Ability: Contrary
EVs: 56 HP / 200 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Leaf Storm
- Hidden Power [Fire]
- Taunt
- Glare
Serperior is able to switch into Leech Seed and Taunt Ferrothorn to prevent paralysis with Thunder Wave or setting up Stealth Rock/Spikes. It's essentially setup fodder with Leaf Storm and once +6 is acheived, Hidden Power Fire finishes it off.
reserving mega charizard y
Charizard @ Charizardite Y
Ability: Blaze
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
- Flamethrower
- Solar Beam
- Scorching Sands
- Roost
Mega Charizard Y is a check to ferrothorn, as it kills it with a super effective flamethrower. It can switch in on most of ferrothorn's moves, but it fears thunder wave.
Offensive calcs:
252 SpA Charizard-Mega-Y Flamethrower vs. 252 HP / 252+ SpD Ferrothorn in Sun: 672-792 (190.9 - 225%) -- guaranteed OHKO
:Defensive calcs:
0 Atk Ferrothorn Knock Off vs. 0 HP / 4 Def Charizard-Mega-Y: 55-65 (18.5 - 21.8%) -- guaranteed 3HKO after Stealth Rock
0 Atk Ferrothorn Gyro Ball (108 BP) vs. 0 HP / 4 Def Charizard-Mega-Y: 67-80 (22.5 - 26.9%) -- 39.5% chance to 2HKO after Stealth Rock
0 Atk Ferrothorn Power Whip vs. 0 HP / 4 Def Charizard-Mega-Y: 37-44 (12.4 - 14.8%) -- 100% chance to 4HKO after Stealth Rock
4 Def Ferrothorn Body Press vs. 0 HP / 4 Def Charizard-Mega-Y: 45-53 (15.1 - 17.8%) -- 25.5% chance to 3HKO after Stealth Rock
Reserving reuniclus(again)
Counter: Calm Mind Reuniclus
Reuniclus @ Fightinium Z / Electrium Z
Ability: Magic Guard
EVs: 252 HP / 212 Def / 44 Spe
Bold Nature
IVs: 0 Atk
- Calm Mind
- Psyshock / Stored Power
- Recover
- Focus Blast / Thunder
Passive damage is ferrothorn's selling point, with knock off amplifying the impact of it, but reuniclus is unphazed by all of the stuff ferrothorn threatens it due to magic guard while happily setting up calm minds. The Z-crystal will prevent knock off from being boosted, so you don't need to click recover as much. Also you can invest in 48 speed evs in case you want to outrun a pex that invested in 4 speed evs.
0 Atk Ferrothorn Knock Off vs. 252 HP / 212+ Def Reuniclus: 80-96 (18.8 - 22.6%) -- possible 5HKO
Worst case scenario: if you ran out of pp on psychic STAB and used the z
+6 0 SpA Reuniclus Thunder vs. 252 HP / 232+ SpD Ferrothorn: 126-148 (35.7 - 42%) -- 89.6% chance to 3HKO after Leftovers recovery
==> ==> most common defensive mega sciz spread 
Using 244 Def and 136 Spatk with a modest nature gets you more spatk out of it without losing any defenseBalloon Mag
Magnezone @ Air Balloon
Ability: Magnet Pull
EVs: 128 Def / 252 SpA / 128 Spe
Bold Nature
- HP Fire
- Thunderbolt
- Volt Switch
- Toxic / Filler
Air Balloon means that even if sci is running sand tomb for pex, mag won't die.
Speed to outspeed 0spe bulky scizor, 252 SpA for max power, rest dumped into def to be able to actually check sci.
252 SpA Magnezone Hidden Power Fire vs. 248 HP / 244 SpD Scizor-Mega: 208-248 (60.6 - 72.3%) -- guaranteed 2HKO
+2 0 Atk Scizor-Mega Knock Off (97.5 BP) vs. 0 HP / 128+ Def Magnezone: 143-169 (50.7 - 59.9%) -- guaranteed 2HKO
As you can see, Mag is a Check to Mega Scizor. Against defensive sets lacking superpower, it can easily take them on by outspeeding and 2HKOing while scizor cannot 2HKO. Against offensive sets, you need to get an aggressive double switch to get in, because if an offensive Scizor hits you on the switch or SDs on the switch, Magnezone cannot check it.252+ Atk Scizor-Mega Superpower vs. 0 HP / 128+ Def Magnezone: 232-274 (82.2 - 97.1%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Magnezone Hidden Power Fire vs. 0 HP / 0 SpD Scizor-Mega: 264-312 (93.9 - 111%) -- 93.8% chance to OHKO
reserving volcarona
Volcarona @ Heavy-Duty Boots
Ability: Flame Body
EVs: 248 HP / 156 Def / 16 SpA / 88 Spe
Timid Nature
- Quiver Dance
- Roost
- Flamethrower
- Psychic/Giga Drain/Bug Buzz/Safeguard/Whatever
Volcarona counters scizor. It switches into bullet punch easily, and threatens a flame body burn on scizor, while being able to KO with Flamethrower and set up with Quiver Dance.
Calcs:
16 SpA Volcarona Flamethrower vs. 248 HP / 244 SpD Scizor-Mega: 408-480 (118.9 - 139.9%) -- guaranteed OHKO
0 Atk Technician Scizor-Mega Bullet Punch vs. 248 HP / 156 Def Volcarona: 53-63 (14.2 - 16.8%) -- possible 6HKO
Reserving Kantonian Zapdos
Defensive Zapdos(standard spread)
Zapdos @ Heavy-Duty Boots
Ability: Static
EVs: 248 HP / 188 Def / 72 Spe
Bold Nature
- Discharge / Hurricane
- Heat Wave
- Roost
- Defog / Hidden Power Ice
No Mega scizor runs max speed, so this standard spread of the electric bird outspeeds it. Heat wave is needed here in order to make sure luck is not needed as much. Also Offensive sets are slightly riskier at doing this job.
most offensive mega sciz possible
+2 252+ Atk Scizor-Mega Knock Off (97.5 BP) vs. 248 HP / 188+ Def Zapdos: 219-258 (57.1 - 67.3%) -- guaranteed 2HKO
+2 252+ Atk Scizor-Mega Knock Off vs. 248 HP / 188+ Def Zapdos: 147-174 (38.3 - 45.4%) -- guaranteed 2HKO after Stealth Rock
+2 252+ Atk Technician Scizor-Mega Bullet Punch vs. 248 HP / 188+ Def Zapdos: 102-120 (26.6 - 31.3%) -- guaranteed 3HKO after Stealth Rock
252+ Atk Technician Scizor-Mega Bullet Punch vs. 248 HP / 188+ Def Zapdos: 51-60 (13.3 - 15.6%) -- possible 5HKO after Stealth Rock
252+ Atk Scizor-Mega Knock Off vs. 248 HP / 188+ Def Zapdos: 74-88 (19.3 - 22.9%) -- guaranteed 4HKO after Stealth Rock
Even after being knocked off mega sciz still needs 3 rounds of double switch with rocks up to pummel zapdos, which shouldn't really happen if you are playing decently well
0 SpA Zapdos Heat Wave vs. 248 HP / 244 SpD Scizor-Mega: 264-312 (76.9 - 90.9%) -- guaranteed 2HKO
0 SpA Zapdos Discharge vs. 248 HP / 244 SpD Scizor-Mega: 84-99 (24.4 - 28.8%) -- guaranteed 4HKO after Stealth Rock
0 SpA Zapdos Hurricane vs. 248 HP / 244 SpD Scizor-Mega: 114-135 (33.2 - 39.3%) -- guaranteed 3HKO after Stealth Rock
Yes that's how bulky mega sciz is. Heat wave doesn't even come close to gurantee kill
Checks
Balloon Mag
Magnezone @ Air Balloon
Ability: Magnet Pull
EVs: 244 Def / 136 SpA / 128 Spe
Modest Nature
- HP Fire
- Thunderbolt
- Volt Switch
- Toxic / Filler
Air Balloon means that even if sci is running sand tomb for pex, mag won't die.
Speed to outspeed 0spe bulky scizor, 252 SpA for max power, rest dumped into def to be able to actually check sci.
Thanks to Hpdsd for tipping me off on a better ev spread!
136+ SpA Magnezone Hidden Power Fire vs. 248 HP / 244 SpD Scizor-Mega: 212-252 (61.8 - 73.4%)
+2 0 Atk Scizor-Mega Knock Off (97.5 BP) vs. 0 HP / 244 Def Magnezone: 143-169 (50.7 - 59.9%)
As you can see, Mag is a Check to Mega Scizor. Against defensive sets lacking superpower, it can easily take them on by outspeeding and 2HKOing while scizor cannot 2HKO. Against offensive sets, you need to get an aggressive double switch to get in, because if an offensive Scizor hits you on the switch or SDs on the switch, Magnezone cannot check it.252+ Atk Scizor-Mega Superpower vs. 0 HP / 244 Def Magnezone: 232-274 (82.5 - 97.5%)
136+ SpA Magnezone Hidden Power Fire vs. 0 HP / 0 SpD Scizor-Mega: 268-316 (95.3 - 112.4%) -- guaranteed OHKO after Stealth Rock
yo mag is a check and volc is a counter
