Moves
Leaf Storm is a very powerful STAB move that lets Mega Sceptile OHKO and 2HKO many Pokemon that its other Grass-type moves cannot. For example, Leaf Storm has a decent chance to 2HKO physically defensive Magic Guard Clefable and OHKO Choice Scarf Landorus-T, Mega Manectric, and Assault Vest Azumarill after Stealth Rock. However, because Leaf Storm tends to force out Mega Sceptile after use because of the Special Attack drop it induces, Energy Ball can be used instead if you want a more consistent STAB move. In addition, Energy Ball does not allow for Pokemon that would otherwise be OHKOed by Leaf Storm, such as Mega Slowbro, to switch in on a Mega Sceptile at -2 Special Attack and abuse the situation. Both moves allow Mega Sceptile to check the bulky Water-types that it's supposed to, such as Rotom-W and Keldeo. Dragon Pulse is a great secondary STAB move that only Fairies are immune to and only Steel-types resist. Additionally, it hits Dragon-type Pokemon super effectively, which allows Mega Sceptile to effectively revenge kill Dragon-types such as Latias and Garchomp. Earthquake can be used to effectively handle Heatran, Klefki, Jirachi, Magnezone, and Tentacruel, and Hidden Power Fire can be used to cover Scizor, Ferrothorn, and Skarmory. One small problem with Hidden Power Fire is that it forces Mega Sceptile to use a Speed IV of 30, which prevents it from Speed tying with Mega Beedrill.
Set Details
Mega Sceptile's base 145 Speed, Lightning Rod ability, and Dragon typing are all integral to its success in OU, so Sceptile needs to be used with Sceptilite. Maximum Speed EVs with a Speed-boosting nature are used in order to get the most of Mega Sceptile's revenge killing ability; if Mega Sceptile is using a Special Attack-boosting nature it loses out on being able to revenge kill Mega Lopunny and Mega Manectric. Even though Hidden Power Fire drops Mega Sceptile's Speed IV to 30, maximum Speed investment should still be used so that you can at least Speed tie other Mega Sceptile with Hidden Power Fire. A Naive nature is chosen on sets with Earthquake so that you don't drop either of Mega Sceptile's offensive stats. If you're using only Focus Blast for coverage against Steel-types, a Timid nature should be used to prevent any unnecessary hindrance to Mega Sceptile's Special Defense. Maximum Special Attack investment is used to ensure it hits as hard as possible.
Usage Tips
Mega Sceptile's base 120 Speed before Mega Evolving is great, but it is not always enough, as it doesn't allow Mega Sceptile to revenge kill Mega Lopunny or Mega Manectric. Thus, it's very important for Mega Sceptile to Mega Evolve early on when playing against offensive teams. One method by which you could bring in Mega Sceptile is with a slow Volt Switch or U-turn; this ensures Mega Sceptile gets in unscathed. After Sceptile Mega Evolves and acquires Lightning Rod, you can have it pivot in on Electric-type attacks from the likes of Thundurus, Mega Manectric, Raikou, and Rotom-W, receive a Special Attack boost, and subsequently begin to use extremely powerful attacks. Do beware that some of these Electric-types may carry Hidden Power Ice, or in Thundurus's case, Hidden Power Flying, which does severe damage to Mega Sceptile. Thanks to its 4x Water resistance, Mega Sceptile can also pivot in on various Water-types, such as Slowbro, Quagsire, Keldeo, and Alomomola, and annihilate them with a STAB Leaf Storm. Again, be aware that some of these Pokemon may carry Ice Beam or Icy Wind, both of which demolish Mega Sceptile. One perk of Mega Sceptile is that, unlike with other fast attackers, you don't have to be cautious when playing against a team with Thundurus, because Mega Sceptile cannot be stopped by Prankster Thunder Wave.
Team Options
Defensively, Steel-types such as Klefki, Bronzong, and Jirachi as well as Poison-types such as Amoonguss, Dragalge, and Gengar can handle the Fairy-types that Mega Sceptile has trouble with, such as Clefable, Mega Altaria, Togekiss, and Sylveon. Togekiss especially gives Mega Sceptile a hard time, so checks to it, such as Rotom-W and Thundurus, make for good teammates. Mega Sceptile hates Talonflame's priority Brave Bird, so Talonflame switch-ins such as Heatran, Rhyperior, and Rotom-W are helpful. Heatran deserves a special mention for also being able to beat the bulky Grass-types, such as Amoonguss, that Mega Sceptile hates.
Offensively, Mega Sceptile makes a good partner for Talonflame as well, as Mega Sceptile beats Rotom-W, Rhyperior, Tyranitar, and Heatran, all Pokemon that counter Talonflame. Gyarados hates Rotom-W for its ability to wall and OHKO it with Volt Switch. Mega Sceptile can OHKO Rotom-W for Gyarados, and if it switches in on a Volt Switch, it will get a Special Attack boost. In return, Gyarados with Bounce beats the bulky Grass-types that Mega Sceptile cannot, such as Amoonguss. As Mega Sceptile struggles greatly against stall, it appreciates powerful wallbreakers to break the walls that it cannot. Choice Specs Sylveon, Choice Specs Dragalge, Swords Dance Diggersby, and Swords Dance Crawdaunt can all help break defensive teams for Mega Sceptile.