CM Clefable beats it, regardless of Unaware or Magic Guard. 28 SA EVs gives Clefable enough Special Attack for Moonblast to 3HKO 62% of the time meaning it can usually win a Calm Mind war. Life Orb MG Clef can easily 3HKO with no investment.
Actually Magic Guard Clefable doesn't beat Mega Bro that easily. The most optimal set is Scald, Ice Beam/Psyshock, Calm Mind and Slack Off imo. And with those last two moves Mega Bro will definitely stall you and setup.
LO Thundurus does 75% min to +1 Slowbro (with a chance of OHKO after Stealth Rock) and can 3HKO even if it is at +5. Non-LO Thunderbolt still does around 65% on average while it survives +1 Scald. Switching in on a +0 Scald and getting burned is pretty much the end of Thundurus though.
It's a special oriented mon... still can't get in on a +1 Mega Bro with Ice Beam.
Sub CM Keldeo's Subs can survive a Scald and at +6 3HKOs with Secret Sword. Switching on a Scald burn is death unfortunately.
Can't really do nothing against Psyshock.
Zapdos does 47.7 - 56.8% with Tbolt, a guaranteed 2HKO with the standard 68 SA EVs or simply Stealth Rocks. +1 Scald can't OHKO even with Stealth Rocks and a burn.
Takes around 46.3 - 54.6% from unboosted Ice Beam.
Magnezone easily 2HKOs. Specs OHKOs even after it Calm Minds.
Special oriented mon, again.
Sub Gyarados sets up on it and easily 2HKOs with Crunch at +1 and 3HKOs at +0. Scald does not break a Sub even at +1.
Both Ice Beam and Psyshock break Sub.
Specs Gothitelle does about as much damage as Mega Venusaur (as mentioned prior, beats it even if it CMs on the switch in, CMs after a hit, Rests, then Sleep Talk picks Calm Mind twice). Unlike the others it doesn't give a shit if it gets burned on a switch in because then it can just use Thunderbolt on +0 cune and take 80% off minimum. While Suicune can't even 2HKO unless it gets a burn on the first hit.
Special oriented mon, again.
Gardevoir easily 2HKOs +1 Slowbro. Scald actually does a fair amount to Gardevoir though so if it takes a Scald on the switch in it will be unable to beat Slowbro on the second switch.
Special oriented mon, again.
Charizard Y easily 2HKOs +1 M Slowbro with Solarbeam and tanks a +1 Scald easily enough.
Special oriented mon, again.
LO Breloom can OHKO after Rocks with 4 hit Bullet Seed, or KO with a 3 hit + a 3 hit after being burned. It can also play Sleep Talk mind games with Spore.
LO Breloom has to hope to get 4/5 hits and not get reduce to his Focus Sash with Ice Beam/Psyshock.
Lum Berry DD Dragonite 2HKOs Slowbro at +2 and has a pretty low chance of getting double burned.
0 SpA Slowbro Ice Beam vs. 216 HP / 228 SpD Multiscale Dragonite: 132-156 (35 - 41.3%) -- guaranteed 3HKO
This is what Mega Slowbro does with a bulky Dragonite.
Gengar 2HKOs with 148 EV Black Sludge Shadow Ball and survives a +1 Scald. With Life Orb and Stealth Rocks Slowbro will walk away with at max 18% of its HP remaining.
Gengar has to be LO max Spe Atk to straight up OHKO, otherwise will get hit by a potencial Psyshock.
CM Latias / Latios both easily break it in a Calm Mind war regardless of what Lati@s EVs are. Mega Lati@s also win.
Special oriented mon, again.
M Altaria can run a bulky DD / Roost / Heal Bell set and basically not give a shit.
0 SpA Slowbro Ice Beam vs. 252 HP / 252+ SpD Altaria: 228-272 (64.4 - 76.8%) -- guaranteed 2HKO
M Sceptile 2HKOs with Giga Drain, OHKOs with Leaf Storm, and can win with SD Leaf Blade, sometimes even if it gets burned!
Special oriented mon, again.
AV Raikou and Mega Manectric both rip into +1 Slowbro.
Special oriented mons, again.
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I don't know why you adopted a Sleep Talk set as the most optimal, but the point is, nobody said Mega Slowbro was unstoppable. Mega Slowbro is one heck of a physical wall, period. It surely can compensate the lack of investment in Spe Def with Calm Mind but specially oriented mons will still reck him. That's what people said here. The only physical mon in OU that can hope to OHKO Mega Slowbro is Breloom and has to be LO and hit 5 times Bullet Seed. That is worth something, right?