Three Chums!

[Chocolate™]

Good morning PC. The name's Chocolate™ and I'm bringing you my newest OU team. This team works around the core of Gliscor, Azumarrill and Mega Manectric. It works surprisingly well and so yeah, five battles later I'm bringing this team to you:

Azumarill @ Sitrus Berry
Ability: Huge Power
EVs: 4 HP / 252 Atk / 252 Spd
Jolly Nature
- Aqua Jet
- Waterfall
- Belly Drum
- Play Rough

Manectric @ Manectite
Ability: Lightningrod
EVs: 252 SAtk / 4 SDef / 252 Spd
Timid Nature
- Thunderbolt
- Overheat
- Volt Switch
- Hidden Power [Ice]

Gliscor @ Toxic Orb
Ability: Poison Heal
EVs: 244 HP / 32 Def / 224 Spd
Impish Nature
- Substitute
- Roost
- Toxic
- Earthquake

Skarmory @ Leftovers
Ability: Sturdy
EVs: 252 HP / 16 Def / 240 SDef
Impish Nature
- Whirlwind
- Roost
- Brave Bird
- Defog

Volcarona @ Leftovers
Ability: Flame Body
EVs: 248 HP / 192 Def / 68 Spd
Bold Nature
- Bug Buzz
- Fiery Dance
- Quiver Dance
- Roost

Diggersby @ Choice Scarf
Ability: Huge Power
EVs: 4 HP / 252 Atk / 252 Spd
Adamant Nature
- Return
- Earthquake
- Wild Charge
- U-Turn

 

[PlatinumDude]

Jolly > Adamant on BellyJet Azumarill (Jolly with 4 HP/252 Atk/252 Spe). Why? Because being outsped by Wash Rotom and being crippled by its Will-o-Wisp before getting the Belly Drum off sucks.

Timid > Modest on Mega Manectric. 135 Speed is good and you might as well take advantage of it. With Modest, you're outsped by Dugtrio, Greninja, Therian Tornadus, Noivern and Alakazam.

What KOs does Gliscor avoid with its current spread? Even with specially defensive investment, Gliscor still falls to decently powered Water moves. The preferred spread is 244 HP/32 Def/224 Spe. While you do lose out on physical bulk, the Speed investment is important for Gliscor to outspeed Mega Garchomp and those slower than it so that it can stall them out before they can do anything to it.

SubLiechi or Power Herb activates Hawlucha's Unburden more reliably. STAB Acrobatics (without an item) still does more to Fairies than a 2x super effective Poison Jab would; outside of Fairies, it's still useless:
-Acrobatics: 55 x 2 x 1.5 = 165 BP
-Poison Jab: 80 x 2 = 160 BP

So in the end:
-Substitute
-Swords Dance
-High Jump Kick
-Acrobatics
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Liechi Berry
Ability: Unburden

or
-Swords Dance
-High Jump Kick
-Acrobatics
-Sky Attack
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Power Herb
Ability: Unburden

Since you have a Pokemon very vulnerable to Stealth Rock in Volcarona, I think you should consider having Mandibuzz, Skarmory or Lati@s as a Defogger to clear away Stealth Rock (over Hawlucha):

Mandibuzz:
-Foul Play
-Defog
-Roost
-Knock Off/Toxic/Whirlwind
Nature: Impish/Bold
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Overcoat

Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Healing Wish/Roost/Recover
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb

Latios:
-Draco Meteor
-Psyshock
-Thunderbolt/Hidden Power (Fighting)/Earthquake
-Recover/Roost/Defog
Nature: Timid/Hasty
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb

Skarmory:
-Whirlwind
-Roost
-Stealth Rock/Spikes/Brave Bird
-Defog/Brave Bird
Nature: Bold/Impish
EVs: 252 HP/252 Def/4 SDef or 252 HP/16 Def/240 SDef
Item: Leftovers
Ability: Sturdy

Bulky Volcarona fares better in the X/Y meta than its offensive counterpart. The extra recovery from Roost helps it out big-time, as it lets it switch into Stealth Rock more than once and the extra bulk lets it set up on opponents it normally can't (like Wash Rotom):
-Quiver Dance
-Bug Buzz
-Fiery Dance
-Roost
Nature: Bold
EVs: 248 HP/192 Def/68 Spe
Item: Leftovers
Ability: Flame Body

What does Hammer Arm cover that Diggersby's other moves can't? Having a strong STAB in Return is important too, you know; also, Wild Charge > Stone Edge to better handle Skarmory and other bulky Waters. And Choice Scarf or Life Orb > Band, since Life Orb Diggersby is strong enough and Diggersby isn't that fast:
-Swords Dance
-Return
-Earthquake
-Quick Attack
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb
Ability: Huge Power

or
-Return
-Earthquake
-Wild Charge
-U-turn
Nature: Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Scarf
Ability: Huge Power

 

[Chocolate™]

I'll be writing my comments into red:

Jolly > Adamant on BellyJet Azumarill (Jolly with 4 HP/252 Atk/252 Spe). Why? Because being outsped by Wash Rotom and being crippled by its Will-o-Wisp before getting the Belly Drum off sucks.

I changed the EV spread and now Azumarrill kicks butt.

Timid > Modest on Mega Manectric. 135 Speed is good and you might as well take advantage of it. With Modest, you're outsped by Dugtrio, Greninja, Therian Tornadus, Noivern and Alakazam.

Thanks for this too.

What KOs does Gliscor avoid with its current spread? Even with specially defensive investment, Gliscor still falls to decently powered Water moves. The preferred spread is 244 HP/32 Def/224 Spe. While you do lose out on physical bulk, the Speed investment is important for Gliscor to outspeed Mega Garchomp and those slower than it so that it can stall them out before they can do anything to it.

Gliscor works great currently.

SubLiechi or Power Herb activates Hawlucha's Unburden more reliably. STAB Acrobatics (without an item) still does more to Fairies than a 2x super effective Poison Jab would; outside of Fairies, it's still useless:
-Acrobatics: 55 x 2 x 1.5 = 165 BP
-Poison Jab: 80 x 2 = 160 BP

So in the end:
-Substitute
-Swords Dance
-High Jump Kick
-Acrobatics
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Liechi Berry
Ability: Unburden

or
-Swords Dance
-High Jump Kick
-Acrobatics
-Sky Attack
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Power Herb
Ability: Unburden

Since you have a Pokemon very vulnerable to Stealth Rock in Volcarona, I think you should consider having Mandibuzz, Skarmory or Lati@s as a Defogger to clear away Stealth Rock (over Hawlucha):

Mandibuzz:
-Foul Play
-Defog
-Roost
-Knock Off/Toxic/Whirlwind
Nature: Impish/Bold
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Overcoat

Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Healing Wish/Roost/Recover
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb

Latios:
-Draco Meteor
-Psyshock
-Thunderbolt/Hidden Power (Fighting)/Earthquake
-Recover/Roost/Defog
Nature: Timid/Hasty
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb

Skarmory:
-Whirlwind
-Roost
-Stealth Rock/Spikes/Brave Bird
-Defog/Brave Bird
Nature: Bold/Impish
EVs: 252 HP/252 Def/4 SDef or 252 HP/16 Def/240 SDef
Item: Leftovers
Ability: Sturdy

Replaced Hawlucha with Skarmory.

Bulky Volcarona fares better in the X/Y meta than its offensive counterpart. The extra recovery from Roost helps it out big-time, as it lets it switch into Stealth Rock more than once and the extra bulk lets it set up on opponents it normally can't (like Wash Rotom):
-Quiver Dance
-Bug Buzz
-Fiery Dance
-Roost
Nature: Bold
EVs: 248 HP/192 Def/68 Spe
Item: Leftovers
Ability: Flame Body

Volcarona is my MVP in this team. He just tanks its, sets up and destroys.

What does Hammer Arm cover that Diggersby's other moves can't? Having a strong STAB in Return is important too, you know; also, Wild Charge > Stone Edge to better handle Skarmory and other bulky Waters. And Choice Scarf or Life Orb > Band, since Life Orb Diggersby is strong enough and Diggersby isn't that fast:
-Swords Dance
-Return
-Earthquake
-Quick Attack
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb
Ability: Huge Power

or
-Return
-Earthquake
-Wild Charge
-U-turn
Nature: Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Scarf
Ability: Huge Power

I'm using the Choice Scarf set which works really well.

Updating the OP accordingly.