(FYI, I'm basing this answer on the assumption that this team is made for the Gen 7 Meta, since this team is made for OU and Mega Mawile is banned to Ubers in Gen 6): Overall, this seems to be a fairly solid team. A nice defensive backbone of Gliscor and Rotom-Wash to support your less than bulky attackers. Entry hazard removal in the form of Gliscor and a solid revenge killer in Hydreigon. However, there are a couple of things holding your team back just a little bit. Volcarona, because of its crippling stealth rock weakness, needs REALLY good entry hazard support. Although Gliscor is a very good Defog user, Gliscor may not live long enough through the match to keep Volcarona safe from the hazard. This is especially crucial because your Volcarona is running Life Orb, further crippling it in the face of stealth rock due to residual damage. Entry hazard removal is also crucial for Azumarill. Because it's main use is going to be Belly Drum, which can't be used if Azumarill gets to half health or below, making sure that you don't take damage just by switching in is a must. Again, you have entry hazard removal in the form of Gliscor, but Gliscor might not be enough. Overall. In my opinion, Azumarill should be dropped since Mega Mawile effectively serves the same purpose and does better in the current meta. Protean Greninja is a pretty good alternative, as it is an overall solid pokemon and can even set up Spikes for the team. Going back to Gliscor, it CAN be used as the special defense part of your backbone, but it probably shouldn't. While I'm not saying that specially defensive Gliscor is bad, I've used one and it actually did fairly well, I'm saying that physically defensive Gliscor would be the better choice because it makes use of Gliscor's higher defensive stat. Next is Hydreigon. Hydreigon, while a very solid revenge killer, is in UU this gen for a reason. Hydreigon simply isn't fast enough, even with a Choice Scarf, to outspeed key threats in the metagame. Such threats include Scarf Blacephalon, Scarf Keldeo, Scarftana and Scarfchomp (yes, these are all Choice Scarf pokemon, but they are very common Scarf users). Beyond that, Hydreigon can't quite match up in terms of power unless it spams Draco Meteor all the time, which leaves it vulnerable to the many fairy types in the metagame. This next problem is with the team as a whole- it's completely walled by one of the most common defensive pokemon in the metagame, Toxapex. Toxapex can tank most any hit that your team can dish out and heal off any damage it takes with either Rexover or by simply switching out because of Regenerator. Gliscor, with the spread it has, can deal up to 59% to it with Earthquake, but Toxapex is usually carrying Black Sludge. Also, that's under the assumption that the opposing team isn't packing a Tapu Bulu. While Tapu Bulu isn't common enough to merit changing the team just because of it, it is still fairly common and puts your team in an even bigger bind. So, with all of this in mind, I'm going to make a list of the changes that can be made to make the team perform better for the gen 7 meta.
(For any changes that require a new party member, I will detail the sets afterwards)
-Give Rotom-Wash Defog (possibly in place of Will-O-Wisp) and change its spread to specially defensive
-Change Gliscior to a physically defensive set
-Drop Azumarill and replace it with Protean Greninja
-Drop Hydreigon and replace it with Tyranitar
And here are the sets: (I will also detail why the sets were chosen)
Greninja @ Life Orb/Waterium Z
EVs: 252 SpA / 4 SpD / 252 Spe
IVs: 0 Atk
- Dark Pulse/Extrasensory
- Hydro Pump/Scald
- Spikes/Toxic Spikes
- Ice Beam/U-Turn
This set should seem pretty self-evident; a Greninja that deals damage to a wide variety of pokemon. Spikes and Toxic Spikes provide good support for the rest of the team and can make your opponent think twice about switching too many times. If running Toxic Spikes, use Extrasensory over Dark Pulse so you can nail Toxapex on the switch when it goes to absorb the Toxic Spikes. Hydro Pump is the preferred water move due to the added power, but Scald is a nice alternative if the unreliable accuracy of Hydro Pump makes you nervous. U-Turn can be used if gaining offensive momentum is important to you.
Tyranitar @ Rockium Z/Smooth Rock
Ability: Sand Stream
EVs: 252 HP / 252 Atk
or 252 Spe / 252 Atk
- Stone Edge
- Dragon Dance/Earthquake
Tyranitar is a solid offensive pokemon that can check a lot of threats in the OU metagame. If getting rid of Toxapex is really important (which it should be), then run Rockium Z and run Pursuit over Thunder Wave. Continental Crush from will deal, at minimum, 75% to Toxapex. If the opponent goes to switch out Toxapex (which they most likely would after Continental Crush), then Pursuit is guaranteed to kill it.
Rotom-Wash @ Leftovers
EVs: 252 HP / 252 SpD / 4 Spe
IVs: 0 Atk
- Hydro Pump
- Volt Switch
- Pain Split
Not much explanation needed, I explained it in the opening ramble. Specially defensive Rotom-Wash has better synergy with physically defensive Gliscor.
Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 252 HP / 184 Def / 72 Spe
Not much explanation needed, I explained most of it in the opening ramble. U-Turn can be used if gaining offensive momentum is important to you.
Again, these suggestions are being made under the assumption that the team is made for Gen 7 OU (since you said this is meant to be an OU team and Mega Mawile is in Ubers in Gen 6).