Check:
Sceptile-Mega @ Sceptilite
Ability: Overgrow
EVs: 4 Atk / 252 SpA / 252 Spe
Rash Nature (or Modest if running pure special, or Naive/Timid if speed is necessary)
- Leaf Storm
- Giga Drain / Rock Slide
- Dragon Pulse
- Earthquake / Hidden Power [Fire] / Rock Slide
Mega Sceptile is the only Pokemon in the tier that guarantees an OHKO vs bulky variants of Manaphy (Tail Glow Rain Dance Sets), a feat that cannot even be guaranteed by unboosted Serperior, Modest Thundurus-T, Scarf Magnezone, Modest Charizard-Y, or Adamant Mega-Medicham.
With access to Grass, Dragon, Fire, Rock, and Ground coverage, as well as a decent Attack stat, Sceptile can also function as a master baiter, luring in would-be checks like Volcarona, Talonflame, Charizard-Y and OHKOing them with Rock Slide, as well as luring mons like Heatran in and 2HKOing it with Earthquake.
With such an offense oriented meta in SPL where Rotom-W is running Scarf to help revenge kill, Tornadus-T running Taunt to deter screens, and where clicking Pursuit is seen more as a liability than an asset, Mega Sceptile can truly put its best foot forward and thrive. It also has excellent synergy with many of the mons performing well this SPL such as Volcanion, Gastrodon, Talonflame, Tyranitar, and Excadrill.
Don't sleep on it
Calcs:
252 SpA Sceptile-Mega Leaf Storm vs. 240 HP / 0 SpD
Manaphy: 458-542 (114.2 - 135.1%) --
guaranteed OHKO
0 Atk Sceptile-Mega Rock Slide vs. 0 HP / 0 Def
Volcarona: 336-396 (108 - 127.3%) --
guaranteed OHKO
0 Atk Sceptile-Mega Rock Slide vs. 0 HP / 4 Def
Talonflame: 312-368 (105 - 123.9%) --
guaranteed OHKO
0 Atk Sceptile-Mega Rock Slide vs. 0 HP / 0 Def
Charizard-Mega-Y: 292-344 (98.3 - 115.8%) --
81.3% chance to OHKO
0 Atk Sceptile-Mega Rock Slide vs. 248 HP / 8 Def
Tornadus-Therian: 140-166 (38.7 - 45.9%) --
guaranteed 2HKO after Stealth Rock (this is just AV Torn, Taunt Torn will take way more from Sceptile's STABs
0 Atk Sceptile-Mega Earthquake vs. 252 HP / 0 Def Heatran: 296-352 (76.6 - 91.1%) -- 25% chance to OHKO after Stealth Rock