altaria needs to be already mega evolved or else it bops to Dragon Pulse, def mzor is OHKOed by hp fire after rocks, meta and clef need to be careful with a bit of prior damage as they are 2HKOed.
Yeah, i wouldn't oppose Mega Sceptile going to B+, as it's an excellent check to Mana/Azu and TankChomp, as well as being very effective against HO.
Sorry AM, this is probably not the discussion you want so I'll keep it quick then respond to something else so it's not a waste of a post.
252 SpA Mega Sceptile Giga Drain vs. 252 HP / 96+ SpD Clefable: 118-141 (29.9 - 35.7%) -- 14.1% chance to 2HKO after Stealth Rock, 3 layers of Spikes, and Leftovers recovery
Idk where you're getting this 2HKO from. Even with Stealth rocks and three layers of spikes it's still a roll with unaware, magic guard doesn't care, at all.
HP fire is... really bad on Mega Sceptile and while it is an option the best set is definitely Sub three attacks with Giga drain, which dismantles certain balance cores, if not for the fact that Clefable is everywhere and Clefable is a counter. Since it's obvious I need to expand a little; Mega Sceptile has poor match up versus all playstyles, as balance, stall and offence has viable
counters all of which are splashable. Lets take balance for example, the most notable example is
clefable, celebi, Mega Scizor, SPdef Talonflame, Hoopa-Unbound, Jirachi, Mew, Tornados-Therian, Mega Venusaur, Mega Altaria, Mega Sableye, Mega Gardevoir, Sylveon are all either
counters or
hard checks. And these aren't uncommon, at all. Three of them are S rank and most of the them are A.
But then you look at offence and how poor Mega Sceptiles match-up is versus them as well, getting zero switch ins, and you could argue that Rotom-W gives it a free switch in, but in reality it volt switches as you switch in and then forces you out, over and over again. Offence have these checks/counters to Mega Sceptile:
Mega Aerodactyl, Mega Alakazam, Mega Metagross, Mega Scizor, Mega Altaria, Mega Charizard Y, Mega Lopunny(soft), Mega Pinsir, Mega Heracross, Mega Gallade, Mega Medicham, Mega Beedril, Landorus-Therian, Kyurem-B, Latias, Weavile, Talonflame, Tornadus-Therian, Azumarill, Klefki, Clefable, Hoopa-Unbound ect.
On stall it's even more walled by Chansey/Clefable/Skarmory so I'm not going to go over listing that.
Hydreigon should also go to B, it's received a ton of competition from Hoopa as a dark wallbreaker, If I rephrase that it would be that it's entirely outclassed as a wallbreaker by Hoopa, it's a good pivot with defences but it loses one roll so I don't think it's B+ atm.