Anyone who uses Mega Diancie in Rock mono and a walrein WITH Frost Breath in an Ice mono aren't going far with those teams. Rock NEEDS Mega Aggron and Choice Specs Walrein....... I'll just let you figure out how bad that is of an option.
Mega Sableye is an auto win against any electric team, Mega Ampharos' base 165 special attack can 2HKO Mega Sableye (and thats the highest special attack stat electric gets), but if even one calm mind is up, a crit is needed to win. Now the other options are hitting it with fairy moves from Dedenne (only electric Pokemon with Play Rough) or Lanturn with dazzling gleam. The argument of don't let it set up doesn't matter here cause alot of the time it uses calm mind with the prankster ability still in use so the first boost can't be stopped.
252+ SpA Mold Breaker Mega Ampharos Thunderbolt vs. +1 252 HP / 0 SpD Mega Sableye: 115-136 (37.8 - 44.7%) -- guaranteed 3HKO
252+ Atk Dedenne Play Rough vs. 252 HP / 252+ Def Mega Sableye: 120-144 (39.4 - 47.3%) -- guaranteed 3HKO
252+ SpA Choice Specs Lanturn Dazzling Gleam vs. +1 252 HP / 0 SpD Mega Sableye: 120-142 (39.4 - 46.7%) -- guaranteed 3HKO
The only way I see Electric managing to beat just Sableye is with Nasty Plot Thundurus, but with Stealth Rock damage Thundurus is 2HKO'd by +1 Shadow Ball, so it would require amazing predicts and luck to get Thundurus set up enough to kill Mega Sableye, if it is even possible. Now thats pretty much 3 types that can't handle one Pokemon, and I'm pretty sure Good Old Smogon Bird got quick banned for doing something like that
Ok let me break down this post because almost everything you say is invalid and I don't understand.
You can either Specs Magnezone, Toxic mega amphy, or calm mind sub Raikou to beat Mega Sableye. It is not at all an auto win also Dedenne is not in the slighest even a bit viable or is Choice Specs Dazzling Gleam Lanturn so I don't know why you would even list those.
Rock doesn't need Mega Aggron huh...? Diance is the most used mega by far sitting at 68.26% while Aggron is at 32.34%. Also can you explain your reasoning why it needs Mega Aggron. Then Walrien with Frost breath is a very popular especially niche for beating Mega Sableye and is on most sets along with surf and Hidden Power Fire to try and deal with Mega Scizor.
Before you post things like this look through http://monotypeps.weebly.com/sprite-gallery.html to actually see the usage of the certain Pokemon. Also if you had been looking through the thread there was a huge discussion about Electric and Mega Sableye and multiple lists showing how it could be beaten, I'm just relisting.