I was so skeptical but it really seems like Mantine might have a solid place at the OU table. It counters shit like Keldeo and Zard Y and Volcanion, which is just unheard of.
It's actually going to be such a good blanket special wall/check. You don't even HAVE to run Defog on it (and I hate running Defog on rock-weak things anyways, I think it's suboptimal because it puts way too much pressure on a defensive pokemon that's already often trying to wall things).
252+ SpA Choice Specs Volcanion Sludge Bomb vs. 252 HP / 16+ SpD Mantine: 108-128 (28.8 - 34.2%) -- 99.9% chance to 3HKO after Stealth Rock and Leftovers recovery
252+ SpA Mega Charizard Y Fire Blast vs. 252 HP / 16+ SpD Mantine in Sun: 116-137 (31 - 36.6%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 240 Def Mantine: 103-122 (27.5 - 32.6%) -- 85.2% chance to 3HKO after Stealth Rock and Leftovers recovery
252 SpA Choice Specs Keldeo Hidden Power Electric vs. 252 HP / 16+ SpD Mantine: 264-312 (70.5 - 83.4%) -- 56.3% chance to OHKO after Stealth Rock
232 SpA Mega Gardevoir Psyshock vs. 252 HP / 240 Def Mantine: 154-183 (41.1 - 48.9%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Life Orb Latios Psyshock vs. 252 HP / 240 Def Mantine: 172-203 (45.9 - 54.2%) -- 3.1% chance to 2HKO after Leftovers recovery
252 SpA Life Orb Latios Draco Meteor vs. 252 HP / 16+ SpD Mantine: 187-220 (50 - 58.8%) -- 70.3% chance to 2HKO after Leftovers recovery
252 SpA Choice Specs Latios Draco Meteor vs. 252 HP / 16+ SpD Mantine: 213-252 (56.9 - 67.3%) -- guaranteed 2HKO after Leftovers recovery
180 SpA Life Orb Tornadus-T Hurricane vs. 252 HP / 16+ SpD Mantine: 133-157 (35.5 - 41.9%) -- 87.3% chance to 3HKO after Leftovers recovery
+3 252 SpA Manaphy Ice Beam vs. 252 HP / 16+ SpD Mantine: 137-162 (36.6 - 43.3%) -- 98.9% chance to 3HKO after Leftovers recovery
+5 252 SpA Manaphy Ice Beam vs. 252 HP / 16+ SpD Mantine: 192-226 (51.3 - 60.4%) -- 91.4% chance to 2HKO after Leftovers recovery
252 SpA Life Orb Gengar Sludge Wave vs. 252 HP / 16+ SpD Mantine: 136-161 (36.3 - 43%) -- 98.2% chance to 3HKO after Leftovers recovery
252 SpA Choice Specs Kyurem Ice Beam vs. 252 HP / 16+ SpD Mantine: 148-175 (39.5 - 46.7%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Choice Specs Kyurem Draco Meteor vs. 252 HP / 16+ SpD Mantine: 213-252 (56.9 - 67.3%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Life Orb Alakazam Psyshock vs. 252 HP / 240 Def Mantine: 175-208 (46.7 - 55.6%) -- 16% chance to 2HKO after Leftovers recovery
252 SpA Mega Manectric Volt Switch vs. 252 HP / 16+ SpD Mantine: 316-376 (84.4 - 100.5%) -- 6.3% chance to OHKO
If you do not have (STAB) Electric Special attacks, you are almost certainly not taking this thing down 1v1.