I've decided to go ahead an compile a list of threats that Milotic can effectively take on this generation, not only as a reference but to kind of have an idea of the kinds of pokemon Milotic will want to challenge:
		
		
	
	Milotic@Leftovers
Calm; 240 HP / 252 Def / 16 SpD
Rest
Sleep Talk
Dragon Tail
Boiling Water
I'll assume ResTalk to provide Milotic with nearly 400 defense while asleep, and when it isn't, Boiling Water patches things up. Bolded pokemon are those that Milotic can take with Marvel Scale that it otherwise couldn't.
Without Marvel Scale
	
	
	
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		- Charizard 252 SpA Timid LO Solar Power HP Grass vs. 391/319 Milotic: (49.62% - 58.82%)
- Ninetales 252 SpA Timid LO +2 Energy Ball vs. 391/319 Milotic: (62.40% - 73.66%)
- Moltres 252 SpA Modest LO Energy Ball vs. 391/319 Milotic: (40.41% - 47.57%)
- Houndoom 252 SpA Timid +2 Dark Pulse vs. 391/319 Milotic: (57.03% - 67.52%)
- Blaziken 252 SpA Rash LO HP Grass vs. 391/319 Milotic: (36.83% - 43.48%)
- Infernape 252 SpA Naive LO Grass Knot vs. 391/319 Milotic: (46.04% - 54.22%)
- Infernape 6 Atk Naive LO Close Combat vs. 391/257 Milotic: (40.92% - 48.59%)
- Magmortar 252 SpA Timid LO Thunderbolt vs. 391/319 Milotic: (49.62% - 58.82%)
- Heatran 252 SpA Timid LO HP Grass vs. 391/319 Milotic: (37.85% - 45.01%)
- Victini 252 SpA Timid LO +1 Thunderbolt vs. 391/319 Milotic: (63.94% - 75.70%)
- Shandera 252 SpA Timid Energy Ball vs. 391/319 Milotic: (35.81% - 42.46%)
- Urugamosu 252 SpA Timid LO +1 Bug Buzz vs. 391/319 Milotic: (56.01% - 65.98%)
- Suicune 252 SpA Modest LO +1 HP Electric vs. 391/319 Milotic: (48.08% - 56.78%)
- Vaporeon 252 SpA Modest +1 HP Electric vs. 391/319 Milotic: (42.46% - 50.13%)
- Starmie 252 SpA Timid LO Thunderbolt vs. 391/319 Milotic: (42.46% - 50.13%)
- Omastar 252 SpA Modest LO +2 Rain Hydro Pump vs. 391/319 Milotic: (72.63% - 85.68%)
- Kingdra 252 SpA Modest +1 Draco Meteor vs. 391/319 Milotic: (57.54% - 67.77%)
- Gorebyss 252 SpA Modest LO +2 Rain Hydro Pump vs. 391/319 Milotic: (72.12% - 84.91%)
- Empoleon 252 SpA Modest +1 Torrent Hydro Pump vs. 391/319 Milotic: (40.92% - 48.34%)
- Empoleon 252 SpA Modest Grass Knot vs. 391/319 Milotic: (40.41% - 48.08%)
- Daikenki 252 SpA Rash LO Grass Knot vs. 391/319 Milotic: (52.17% - 61.38%)
- Jirachi 6 SpA Timid +1 Thunderbolt vs. 391/319 Milotic: (38.87% - 46.04%)
- Lucario 252 SpA Timid LO +2 Aura Sphere vs. 391/319 Milotic: (66.24% - 78.26%)
- Genusekuto 252 SpA Timid +1 Thunderbolt vs. 391/319 Milotic: (40.66% - 47.83%)
- Kyuremu 252 SpA Naive LO Draco Meteor vs. 391/319 Milotic: (56.27% - 66.75%)
- Yanmega 252 SpA Timid LO Bug Buzz vs. 391/319 Milotic: (33.76% - 39.90%)
- Zoroark 252 SpA Timid LO +2 Night Burst vs. 391/319 Milotic: (64.45% - 75.96%)
- Sazandora 252 SpA Timid +1 Draco Meteor vs. 391/319 Milotic: (62.92% - 74.42%)
- Latios 252 SpA Timid LO Draco Meteor vs. 391/319 Milotic: (56.27% - 66.75%)
- Salamence 252 SpA Naive LO Draco Meteor vs. 391/319 Milotic: (49.87% - 59.08%)
	 
 
Notice that the majority of these are special hits; this is just to prove that Milotic is actually a pretty capable sponge on the special side. Now for physical hits (all assume Marvel Scale):
	
	
	
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		- Charizard 252 Atk Jolly +6 Earthquake vs. 391/257 +1 Milotic: (50.90% - 60.10%)
- [B]Arcanine[/B] 252 Atk Adamant +1 Wild Volt vs. 391/257 +1 Milotic: (45.52% - 53.71%)
- [B]Blaziken[/B] 60 Atk Rash LO Hi Jump Kick vs. 391/257 +1 Milotic: (35.29% - 41.69%)
- Blaziken 252 Atk Jolly LO +2 Hi Jump Kick vs. 391/257 +1 Milotic: (82.10% - 96.68%)
- [B]Infernape[/B] 252 Atk Naive LO +2 Close Combat vs. 391/257 +1 Milotic: (68.29% - 80.56%)
- Emboar 252 Atk Brave LO Reckless Wild Volt vs. 391/257 +1 Milotic: (58.31% - 69.05%)
- Hihidaruma 252 Atk Jolly CB Superpower vs. 391/257 +1 Milotic: (32.48% - 38.36%)
- Cloyster 252 Atk Jolly LO +2 5-Hit Rock Blast vs. 391/257 +1 Milotic: (44.76% - 52.69%)
- Gyarados 252 Atk Adamant LO +1 Bounce vs. 391/257 +1 Milotic: (45.52% - 53.71%)
- [B]Kabutops[/B] 252 Atk Adamant LO +2 Stone Edge vs. 391/257 +1 Milotic: (67.01% - 79.03%)
- Azumarill 252 Atk Adamant CB Double-Edge vs. 391/257 +1 Milotic: (37.60% - 44.25%)
- Kingdra 252 Atk Adamant LO +1 Outrage vs. 391/257 +1 Milotic: (52.94% - 62.40%)
- Kerudio 252 SpA Timid LO +1 Sword of Mystery vs. 391/257 +1 Milotic: (42.46% - 50.13%)
- Scizor 252 Atk Adamant CB U-Turn vs. 391/257 +1 Milotic: (29.92% - 35.29%)
- Aggron 252 Atk Adamant LO Head Smash vs. 391/257 +1 Milotic: (48.59% - 57.54%)
- [B]Metagross[/B] 252 Atk Adamant CB Explosion vs. 391/257 +1 Milotic: (72.38% - 85.17%)
- [B]Lucario[/B] 252 Atk Adamant LO +2 Close Combat vs. 391/257 +1 Milotic: (78.26% - 92.07%)
- [B]Doryuuzu[/B] 252 Atk Adamant LO +2 Earthquake vs. 391/257 +1 Milotic: (74.62% - 87.82%)
- Mamoswine 252 Atk Jolly CB Earthquake vs. 391/257 +1 Milotic: (38.62% - 45.52%)
- Tyranitar 252 Atk Adamant CB Stone Edge vs. 391/257 +1 Milotic: (43.22% - 50.90%)
- Salamence 252 Atk Naive LO +1 Outrage vs. 391/257 +1 Milotic: (61.38% - 72.38%)
- Garchomp 252 Atk Jolly +2 Outrage vs. 391/257 +1 Milotic: (61.38% - 72.38%)
- [B]Ononokusu[/B] 252 Atk Jolly +2 LO Outrage vs. 391/257 +1 Milotic: (87.47% - 103.07%)
- Ononokusu 252 Atk Adamant +1 LO Outrage vs. 391/257 +1 Milotic: (72.12% - 85.17%)
- [B]Heracross[/B] 252 Atk Adamant +1 CB Megahorn vs. 391/257 +1 Milotic: (73.66% - 86.96%)
- [B]Terakion[/B] 252 Atk Jolly +2 LO Close Combat vs. 391/257 +1 Milotic: (79.28% - 93.61%)
- Terakion 252 Atk Jolly +2 LO Sacred Sword vs. 391/257 Milotic: (89.26% - 105.37%)
- Randorosu 252 Atk Jolly +2 LO Non-Sand Earthquake vs. 391/257 +1 Milotic: (64.72% - 76.14%)
- Randorosu 252 Atk Adamant LO Sand Earthquake vs. 391/257 +1 Milotic: (52.54% - 61.93%)
	 
 
Ononokusu 252 Atk Jolly +2 LO Outrage vs. 416/302 Lugia: (104.81% - 123.56%)
Basically, Milotic is bulkier than Gen 5 Lugia without the Stealth Rock weakness, and admittedly lower SpD. Threats that Milotic can check with Marvel Scale activated include:
-Adamant CB Arcanine
-Jolly DD Mence
-Jolly SD Chomp
-Adamant DD Ononokusu (Basically DDRay)
-Jolly SD Blaziken
-Rash MixKen
-Jolly SD Terakion
-Rain Sweepers (DD/MixDra, Adamant SD Kabutops, Modest Shell Break Gorebyss/Omastar)
-Sand Sweepers (CB/DD AdamantTar, Adamant SD Dory, Adamant RP SPower Randorosu)
-SD Luke / NP Luke
-Urugamosu
-Kerudio
-MixMence
Those are some of the metagame's top threats (Dory, Garchomp, Terakion, Urugamosu, Kerudio, Genusekuto), at least currently, and Milotic is able to check them all in one package, either phazing them or flat out attacking (with a potential burn side effect no less). Dragon Tail allows Milotic to deal with all of these rather than choose (aka Ice Beam vs. HP Electric vs. Toxic vs. Haze, etc). Be sure to pack a grass or electric type.