The first thing to notice is Mega Sceptile's 4x weakness to Ice. Yes, it's fast, but any decently strong Ice Shard user should KO it in one hit. Examples are Weavile, Mamoswine, and Cloyster.
However, Sceptile's limited offensive movepool can also be used against it. Grass and Dragon get walled by opposing Steel types, so something like Corviknight or Skarmory, who are immune to Earthquake, should wall Mega Sceptile and strike back with Brave Bird. Other bulky Pokemon with super effective STAB, like Mega Scizor and Mega Altaria, can easily tank a hit and hit back for massive damage.
Even Pokemon who don't resists Sceptile's STABs can just nuke Mega Sceptile. Take Mega Charizard Y, whose sun boosted Weather Ball annihilates offensive Mega Sceptile while only fearing Rock Slide (which is not listed on this set.) Another good example is Tapu Lele, who can tank a Leaf Storm and OHKO with Moonblast. The same logic goes with Dragonite, who can eat a Dragon Pulse with Multiscale intact and strike back with an Ice Spinner, Dual Wingbeat, or Outrage.
Finally, in the worse case scenario where none of the above can be applied, you can even enlist defensive options to take it on. Magic Guard Clefable will take large damage from the initial Leaf Storm, but it can easily heal off the damage and chip away with Moonblast after Sceptile's Special Attack is lowered. With this set, Pokemon like Eviolite Chansey take about 60% from +6 Earthquake, and even if we assume 252 Attack EVs and Dragon Claw instead of Pulse, Mega Sceptile will still need 3 Swords Dances to have a chance at OHKOing Chansey. At that point, you can simply Toxic, Seismic Toss twice, and effectively trade Chansey for Mega Sceptile. Lastly, Foul Play Pokemon can take advantage of Sceptile's Swords Dance boosts. Mandibuzz, a Pokemon that would otherwise crumble to boosted Sceptile, can easily OHKO it after one Swords Dance boost with Foul Play.
To recap:
1. Strong Ice Shard users that can immediately OHKO Mega Sceptile
2. Strong attackers that can resist/live a hit and OHKO
3. In the worst case scenario, defensive Pokemon that can tank hits with options like Toxic, Seismic Toss, and Foul Play
Hope I helped!
