Also, for the 4-attacks lure set, there is actually a decent amount of stuff Ampharos can survive from defensive/support pokemon after taking a strong attack (basically, stuff it can switch in on for wish support, or an attack it can take to fire off a rest if you're using that set instead). Here are a few calcs of defensive pokemon trying to do their best damage to Ampharos:
0 Atk Forretress Rapid Spin vs. 252 HP / 252+ Def Mega Ampharos: 10-12 (2.6 - 3.1%) because it beats Gyro Ball and Volt Switch lol
4 Atk Ferrothorn Gyro Ball (41 BP) vs. 252 HP / 252+ Def Mega Ampharos: 15-18 (3.9 - 4.6%)
4 Atk Gourgeist-Super Shadow Sneak vs. 252 HP / 252+ Def Mega Ampharos: 31-37 (8 - 9.6%)
4 Atk Mandibuzz Knock Off vs. 252 HP / 252+ Def Mega Ampharos: 34-42 (8.8 - 10.9%)
4 Atk Klefki Foul Play vs. 252 HP / 252+ Def Mega Ampharos: 36-43 (9.3 - 11.1%)
0 Atk Skarmory Brave Bird vs. 252 HP / 252+ Def Mega Ampharos: 38-45 (9.8 - 11.7%)
252 Atk Jirachi Iron Head vs. 252 HP / 252+ Def Mega Ampharos: 38-45 (9.8 - 11.7%)
4 Atk Crobat Brave Bird vs. 252 HP / 252+ Def Mega Ampharos: 42-49 (10.9 - 12.7%)
4 Atk Ferrothorn Power Whip vs. 252 HP / 252+ Def Mega Ampharos: 42-51 (10.9 - 13.2%)
4 Atk Mandibuzz/Sableye/Umbreon/etc Dark type Foul Play vs. 252 HP / 252+ Def Mega Ampharos: 54-64 (14 - 16.6%) -- possible 6HKO
4 SpA Heatran Flamethrower vs. 252 HP / 4 SpD Mega Ampharos: 56-66 (14.5 - 17.1%) careful for Earth Power though
4 SpA Rotom-W Hydro Pump vs. 252 HP / 4 SpD Mega Ampharos: 57-67 (14.8 - 17.4%)
4 Atk Trevenant Shadow Claw vs. 252 HP / 252+ Def Mega Ampharos: 58-69 (15.1 - 17.9%)
4 SpA Togekiss Aura Sphere vs. 252 HP / 4 SpD Mega Ampharos: 62-74 (16.1 - 19.2%) Careful for Dazzling Gleam
252+ Atk Sharp Beak Talonflame Brave Bird vs. 252 HP / 252+ Def Mega Ampharos: 66-78 (17.1 - 20.3%)
4 SpA Jellicent Shadow Ball vs. 252 HP / 4 SpD Mega Ampharos: 70-84 (18.2 - 21.8%)
0 SpA Tentacruel Sludge Bomb vs. 252 HP / 4 SpD Mega Ampharos: 75-88 (19.5 - 22.9%)
252 Atk Jirachi Zen Headbutt vs. 252 HP / 252+ Def Mega Ampharos: 76-91 (19.7 - 23.6%)
252+ Atk Malamar Superpower vs. 252 HP / 252+ Def Mega Ampharos: 79-94 (20.5 - 24.4%)
252+ SpA Togekiss Aura Sphere vs. 252 HP / 4 SpD Mega Ampharos: 84-99 (21.8 - 25.7%) Careful for Dazzling Gleam again, this is a more offensive Togekiss
0 SpA Celebi Psychic vs. 252 HP / 4 SpD Mega Ampharos: 90-106 (23.4 - 27.6%) Careful for Earth Power, or, if you're weakened, offensive versions in general
Chansey Seismic Toss vs. 252 HP Mega Ampharos: 100-100 (26 - 26%)
0 SpA Blissey Ice Beam vs. 252 HP / 4 SpD Mega Ampharos: 94-112 (24.4 - 29.1%)
0 SpA Deoxys-D Psycho Boost vs. 252 HP / 4 SpD Mega Ampharos: 96-114 (25 - 29.6%) Assuming it was nerfed down to 130 base power, like its variations, though I'm not sure if that was confirmed yet
252 Atk Jirachi Ice Punch vs. 252 HP / 252+ Def Mega Ampharos: 96-114 (25 - 29.6%)
0 SpA Mega Venusaur Sludge Bomb vs. 252 HP / 4 SpD Mega Ampharos: 106-126 (27.6 - 32.8%) A little bit tougher to take, especially if it poisons, but if you can predict a Giga Drain you're off easy
BEAUTIFUL!
Thought I'd throw in a few more: support pokemon to M-Amphy. Remember, these are support pokemon, not sweepers of any kind, but not necessarily defensive. Pokemon used as Pivots, as Status, or something in between attack and defense.
252+ SpA Rotom-W Hydro Pump (not the usual set either) vs. 252 HP / 4 SpD Mega Ampharos: 78-92 (20.3 - 23.9%) -- guaranteed 5HKO
0 Atk Gliscor Earthquake vs. 252 HP / 252+ Def Mega Ampharos: 146-174 (38 - 45.3%) -- guaranteed 3HKO
252 SpA Gengar Sludge Bomb (Sub/Disable) vs. 252 HP / 4 SpD Mega Ampharos: 135-160 (35.1 - 41.6%) -- guaranteed 3HKO
4 SpA Heatran Earth Power vs. 252 HP / 4 SpD Mega Ampharos: 150-178 (39 - 46.3%) -- guaranteed 3HKO
252+ SpA Choice Specs Magnezone Flash Cannon vs. 252 HP / 4 SpD Mega Ampharos: 99-117 (25.7 - 30.4%) -- guaranteed 4HKO
252+ Atk Tyranitar Stone Edge vs. 252 HP / 252+ Def Mega Ampharos: 127-151 (33 - 39.3%) -- 99.9% chance to 3HKO
252 SpA Starmie Ice Beam vs. 252 HP / 4 SpD Mega Ampharos: 150-178 (39 - 46.3%) -- guaranteed 3HKO
252 SpA Espeon Psychic vs. 252 HP / 4 SpD Mega Ampharos: 135-160 (35.1 - 41.6%) -- guaranteed 3HKO
I think as we can see, even with little or no special defensive investment, Mega-Amphy sponges these hits pretty well. Of course, these are not hits it wants to take regularly, but it hits back hard, even with no special attack investment either. Coverage and investment in bulk makes Defensive Amphy a real pain in the butt to deal with.