Updated post with new line-up and two showndown videos.
My team really suffers from not having enough fire attacks and against bulky pure water types like Milotic (I had nothing to counter!)
Ferrothorn counters nearly every Water Pokemon well enough. It doesn't need Toxic, as Leech Seed already provides residual damage.
Milotic is irrelevant in OU because it's outclassed by other bulky Waters there.
As I mentioned earlier, get rid of Gyarados' Ice Fang. Mega Gyarados makes better use of it because of Mold Breaker; it 2HKOs non-physically defensive Mega Venusaur and OHKOs Multiscale Dragonite.
Rhyperior's only purpose is to serve as a check/counter to Talonflame. It doesn't make a good Assault Vest user because of its common weaknesses to Water and Grass, which tend to be Special moves. Conkeldurr is the better Assault Vest user to use:
-Drain Punch
-Mach Punch
-Knock Off
-Ice Punch/Stone Edge
Nature: Adamant
EVs: 252 Atk/236 SDef/20 Spe
Item: Assault Vest
Ability: Guts
Aggron should only play a defensive role on the team, but you already have Ferrothorn as a defensive Steel. I'd ditch it for
[email protected] (Latias or Latios), as they're stronger than Noivern (more on that later) and provides Defog support for Gyarados:
Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Healing Wish
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Latios:
-Draco Meteor
-Psyshock
-Thunderbolt/Hidden Power (Fighting)/Earthquake
-Defog
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb
Noivern has to go. It's outclassed by
[email protected], as well as Charizard X, Garchomp and Dragonite. It also shares an unwanted 4x Ice weakness with Garchomp. As a backup speedy special attacker, look no further than Greninja. See my first post on this thread for its move set. Or you can use Charizard Y, Mega Manectric or Mega Gardevoir, as I suggested you remove Aggron from the team:
Charizard Y:
-Fire Blast
-Solar Beam
-Focus Blast
-Roost/Dragon Pulse
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Charizardite Y
Manectric:
-Thunderbolt
-Volt Switch
-Overheat/Flamethrower
-Hidden Power (Ice)
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Manectite
Ability: Lightning Rod
Gardevoir:
-Psyshock
-Hyper Voice
-Focus Blast
-Shadow Ball/Taunt/Will-o-Wisp
Nature: Timid
EVs: 24 Def/232 SAtk/252 Spe
Item: Gardevoirite
Ability: Trace
or
-Calm Mind
-Psyshock
-Hyper Voice
-Focus Blast/Shadow Ball/Taunt
Nature: Timid
EVs: 24 Def/232 SAtk/252 Spe
Item: Gardevoirite
Ability: Trace