Why would Mega Manectric be switching into Milo? This thing has no business switching directly into special attackers in fact, let alone Competitive/Defiant abusers.
I ran calcs against Manectric because the guy brought it up in his list of things that "destroy it outright" I used a max speed, max SpA timid set for Milotic against the standard Mega Man, as timid was one of the sets that got thrown around when people were trying out competitive.
+2 252 SpA Milotic Hydro Pump vs. 0 HP / 0 SpD Mega Manectric: 360-424 (128.1 - 150.8%) -- guaranteed OHKO
252 SpA Mega Manectric Thunderbolt vs. 4 HP / 0 SpD Milotic: 252-296 (75.9 - 89.1%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Mega Manectric Volt Switch vs. 4 HP / 0 SpD Milotic: 194-230 (58.4 - 69.2%) -- guaranteed 2HKO after Leftovers recovery
Not only can it not switch into this variant, but it can't revenge either.
What set are you proposing then? You are being somewhat ambiguous on the moves for the Milotic you are proposing. I just don't see the worth of using Milotic over other waters.
I'm actually using offensive variants for these cause I remember that getting thrown around a lot in early XY. The set would basically be Hydro Pump, Blizzard/Ice Beam, Recover, and Coil. Scald is also an option, but the whole point of running coil would be to have both bulk and to use your stronger moves without worry of missing. Since I'm posting calcs now, might as well run up those defogger calcs. Assuming rocks are what's being defogged away of course (I neglected this in my first calcs)
+2 252 SpA Milotic Hydro Pump vs. 252 HP / 4 SpD Skarmory: 399-471 (119.4 - 141%) -- guaranteed OHKO
+2 252 SpA Milotic Ice Beam vs. 248 HP / 108 SpD Mandibuzz: 306-360 (72.3 - 85.1%) -- 75% chance to OHKO after Stealth Rock
+2 252 SpA Milotic Blizzard vs. 248 HP / 108 SpD Mandibuzz: 374-440 (88.4 - 104%) -- guaranteed OHKO after Stealth Rock
+2 252 SpA Milotic Hydro Pump vs. 248 HP / 108 SpD Mandibuzz: 280-330 (66.1 - 78%) -- 25% chance to OHKO after Stealth Rock
+2 252 SpA Milotic Blizzard vs. 252 HP / 0 SpD Zapdos: 436-514 (113.5 - 133.8%) -- guaranteed OHKO
+2 252 SpA Milotic Ice Beam vs. 252 HP / 0 SpD Zapdos: 358-422 (93.2 - 109.8%) -- guaranteed OHKO after Stealth Rock
+2 252 SpA Milotic Hydro Pump vs. 252 HP / 0 SpD Zapdos: 327-385 (85.1 - 100.2%) -- guaranteed OHKO after Stealth Rock
68 SpA Zapdos Thunderbolt vs. 4 HP / 0 SpD Milotic: 206-246 (62 - 74%) -- guaranteed 2HKO after Leftovers recovery
68 SpA Zapdos Thunder vs. 4 HP / 0 SpD Milotic: 252-296 (75.9 - 89.1%) -- guaranteed 2HKO after Leftovers recovery
The Zapdos one is risky, but if you can predict it right, you can scare him out if not kill him.
Against some of those other things in that "outright destroy" list. Specs Raikou can't switch into a boosted Hydro Pump, or it dies, but otherwise it wins. Mega Venusaur actually loses to boosted Modest variants if rocks are up. Both otherwise he wins that one. Mega Sceptile can't switch in at all and needs to be running Leaf Storm in order to revenge kill. Gengar is 2HKOd by an unboosted Hydro Pump, and I have no idea what he plans to do back (the calc only pulled up Shadow Ball) Charizard Y can't switch into Hydro Pump or he fails to get the 2HKO.
Most of those set up sweepers can't switch into boosted variants and can't set up against haze variants running the old sets. I'm not going to remotely argue that its top tier material, but I think it can pull enough of it's own weight to be D rank. Hell, I haven't even gotten into coil boosts yet. Most of this is just the competitive ability in general.