Sceptile has a number of problems. First, any priority abuser besides Aqua Jet / Water Shuriken users outspeed and likely high damage to it. Second scarfers do the same providing they're fast enough (anything over base 80 positive) and have their choice of move to use. Third, and probably most importantly, it's set up bait for anything that either boosts speed or has priority. If you're clicking Electrify your opponent is free to Dragon Dance or whatever. I don't think Electrify is nearly problematic enough to ban flat out, especially since I have yet to see high level players using the strategy very much.
EDIT: Thought I'd post some answers both defensive and offensive to Electrify Mega Sceptile. It's worth noting that not all of these will beat all of Mega Sceptile's sets, but that's mostly because of the nature of Sketchmons, where every Pokemon COULD be running that one thing that beats you. Some things that MScept could run that are worth considering are Tail Glow, Blue Flare, Earth Power, and Secret Sword.
Muk-Alola @ Black Sludge
Ability: Poison Touch
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Slack Off
- Knock Off / Pursuit
- Poison Jab / Pursuit
- Shadow Sneak
+1 252 SpA Sceptile-Mega Dragon Pulse vs. 252 HP / 252+ SpD Muk-Alola: 162-192 (39.1 - 46.3%) -- guaranteed 3HKO after Stealth Rock and Black Sludge recovery
Even at +1 this Muk-Alola comfortably walls Electrify Mega Sceptile, while also acting as a handy Psychic check. Shadow Sneak allows you to avoid Electrify completely, but gives you a bit of 4mss (you really want dual STAB, recovery, Pursuit, and Shadow Sneak, and maybe Ice Punch too for ground types). You could also run Assault Vest + U-Turn which takes less but gets worn down more easily.
Landorus-Therian (M) @ Choice Scarf
Ability: Intimidate
EVs: 244 Atk / 68 Def / 196 Spe
Jolly Nature
- Dragon Ascent
- U-turn
- Earthquake
- Defog
While it can't really switch into Mega Sceptile (252 SpA Sceptile-Mega Leaf Storm vs. 0 HP / 0 SpD Landorus-Therian: 277-327 (86.8 - 102.5%) -- 18.8% chance to OHKO) Landorus-Therian can revenge kill it with ease. EVs stolen from OU analysis but there are probably better ones for Sketchmons.
Magearna @ Fairium Z
Ability: Soul-Heart
EVs: 252 HP / 252 SpA / 4 SpD
Quiet Nature
IVs: 0 Atk / 0 Spe
- Trick Room
- Fleur Cannon / Moonblast
- Volt Switch
- Secret Sword / Tail Glow / Roost / Calm Mind / Pain Split / one of Magearna's million other options who even cares
Sets Trick Room in Sceptile's face, then blasts it out of existence.
Kyurem-Black @ Choice Scarf
Ability: Teravolt
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Icicle Spear
- Fusion Bolt
- Outrage
- Iron Head
This set is notable for messing with a Sceptile behind a sub better than most other offensive answers. Mamoswine can do something similar. Don't try this without the scarf, Dragon Pulse has a chance to OHKO you.
Chansey (F) @ Eviolite
Ability: Natural Cure
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Soft-Boiled
- Seismic Toss
- Haze
- Toxic / Heal Bell / Stealth Rock / Thunder Wave
This is Chansey. You know what it does. Haze is there to remove any boosts Sceptile may accumulate through sloppy play. It doesn't do much back but it doesn't really need to. Watch out for Secret Sword.