Ace Trainer?

[Empyrean]

Volcarona [Tigrinum] @ Rocky Helmet
Flame Body
Psychic
Bug Buzz
Fiery Dance
Quiver Dance

Lilligant [Dianthus] @ Big Root
Own Tempo
Petal Dance
Dream Eater
Quiver Dance
Sleep Powder

Vaporeon [Nereid] @ Leftovers
Water Absorb
Scald
Ice Beam
Hyper Beam
Shadow Ball

Latias [Bianca] @ Wise Glasses
Levitate
Surf
Psychic
Thunderbolt
Dragon Pulse

Lucario [Anubis] @ Expert Belt
Inner Focus
Earthquake
Aura Sphere
Flash Cannon
Extremespeed

Meloetta [Thalia] @ Sitrus Berry
Serene Grace
Psychic
Acrobatics
Relic Song
Close Combat

... Well? What's the verdict?

 

[Ho-Oh]

Uh, was this made in-game or on Pokemon Online? If it's in-game we're probably going to need levels (only assuming so because of Hyper Beam - which isn't good due to leaving to immobile for one turn and doesn't do the greatest damage in comparison to STAB attacks and LO boosts etc overall) aaand natures/EVs will also help, especially for things like Lucario which can be special or physical and your natures etc may not fit.

Anyway just going to add in a few things, consider Hidden Power (rock or fire) over Dream Eater, since Petal Dance isn't necessarily destroy everything alone - and the HP loss from that isn't really worth it, plus the fact that Sleep Powder itself is unreliable. If it is in-game then that's okay, but it's something to be weary of, since you'll have issues with grass/flying/bugs etc.

Vaporeon is kinda all over the place. With Scald & its general movesets it mostly abuses the hidden ability, Hydration, however yours doesn't really serve any kind of purpose. You could try acid armor/baton pass/substitute with scald to help others with not the greatest defenses or things with double weaknesses, like Lucario, and with things like Terrakion running around a defense boost or three can be useful.

Latias doesn't really need Psychic (or rather Psyshock would be better for fighting types but it can otherwise kill with dragon pulse) so you could replace that with calm mind to further the great defenses and boost spa anyway.

Also on Lucario, EQ is usually redundant when you have a fighting attack w/ STAB that can hit pretty much the same things. However, it would be to your benefit to really specialise in either special or physical, so you could utilise nasty plot, swords dance, choice specs, choice band, etc. So yeah, natures will definitely help that.

But ya, also don't really know much of Meloetta, and it's also unreleased currently, so keep that in mind that people may not want to battle it/you for that reason.

 

[PlatinumDude]

Also, we need to see your EVs. I'll assume this is a PO team.

To back up on what Nica said about Vaporeon, it's better off supporting the team with Wish:
-Wish
-Scald
-Protect
-Ice Beam/Toxic
Nature: Bold
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Water Absorb

While Own Tempo + Petal Dance is tempting, Lilligant is better off using Giga Drain as its STAB because Grass is a pretty poor offensive type. Lilligant isn't a good choice in OU too because of its very shallow movepool. You're better off using Ferrothorn, who has a better movepool and can support the team:
-Spikes/Stealth Rock
-Leech Seed
-Power Whip/Protect
-Gyro Ball/Thunder Wave/Protect
Nature: Relaxed
EVs: 252 HP/88 Def/168 SDef
Item: Leftovers

Latios is better offensively than Latias. If you want to continue using Latias, use this:
-Calm Mind
-Dragon Pulse
-Recover
-Hidden Power (Fire)/Substitute/Roar
Nature: Timid
EVs: 252 HP/4 SAtk/252 Spe
Item: Leftovers

For Lucario, you can choose between these sets:
-Swords Dance
-Close Combat
-Crunch
-ExtremeSpeed/Ice Punch/Stone Edge
Nature: Adamant/Jolly
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb

or
-Nasty Plot
-Aura Sphere
-Dark Pulse/Shadow Ball
-Hidden Power (Ice)/Vacuum Wave
Nature: Modest/Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb

Meloetta hasn't been officially released yet. You have to find something else to take that slot. I'll let other people decide on that.

 

[Empyrean]

Thank you, Platinum Dude. I will revise my team accordingly.

In truth, I only recently got access to WiFi, so I am new to competitive battling- it seems it shows ^^'

Shall I presume my Volcarona alright?

 

[PlatinumDude]

Thank you, Platinum Dude. I will revise my team accordingly.

In truth, I only recently got access to WiFi, so I am new to competitive battling- it seems it shows ^^'

Shall I presume my Volcarona alright?

Not really; just change the item to Life Orb and you're all set.

 

[Empyrean]

Perhaps Gardevoir would make a good replacement for Meloetta? And Haxorus for Lucario...

 

[PlatinumDude]

Perhaps Gardevoir would make a good replacement for Meloetta? And Haxorus for Lucario...

Generally, Alakazam or Reuniclus would be better replacements. Both share the Magic Guard ability, so that's good for protection against sandstorm, Toxic/burn damage and Life Orb recoil. Here are a couple sets you should use if you go with either one:

Alakazam:
-Calm Mind/Substitute
-Psychic/Psyshock
-Shadow Ball
-Focus Blast
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item Life Orb/Focus Sash
Ability: Magic Guard

Reuniclus:
-Calm Mind
-Psychic/Psyshock
-Focus Blast
-Recover
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item Life Orb/Leftovers
Ability: Magic Guard

Why would you replace Lucario, though?

 

[Empyrean]

(Modest) Volcarona [Tigrinum] @ Life Orb
Flame Body
Psychic
Bug Buzz
Fiery Dance
Quiver Dance

(Relaxed) Ferrothorn [Tribulus] @ Rocky Helmet
Iron Barbs
Protect
Gyro Ball
Leech Seed
Stealth Rock

(Modest) Vaporeon [Nereid] @ Leftovers
Water Absorb
Scald
Protect
Ice Beam
Hidden Power (Grass)

(Timid) Latias [Bianca] @ Soul Dew
Levitate
Toxic
Recover
Substitute
Dragon Pulse

(Adamant) Lucario [Anubis] @ Life Orb
Inner Focus
Earthquake
Close Combat
ExtremeSpeed
Swords Dance

(Bold) Reuniclus [Homunculus] @ Life Orb
Magic Guard
Psychic
Recover
Calm Mind
Focus Blast

How's that?