There's no need to say a Pokemon's individual stats. EVs and natures are all that matter.
Absol is only good in its Mega Evolution. Its regular form is mediocre, save for a massive Attack:
-Sucker Punch
-Pursuit/Night Slash/Knock Off
-Fire Blast
-Play Rough/Superpower
Nature: Naive
EVs: 252 Atk/4 SAtk/252 Spe
Item: Absolite
Ability: Super Luck/Justified
or
-Swords Dance
-Sucker Punch
-Night Slash/Knock Off
-Play Rough/Baton Pass
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Absolite
Ability: Super Luck/Justified
Since you already have a Mega Pokemon in Charizard, I'd replace Absol entirely with Bisharp or Mandibuzz:
Bisharp:
-Sucker Punch
-Iron Head
-Knock Off
-Pursuit/Swords Dance
Nature: Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Black Glasses/Dread Plate/Life Orb
Ability: Defiant
Mandibuzz:
-Foul Play
-Defog
-Roost
-Toxic/Whirlwind/Knock Off
Nature: Bold/Impish
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Overcoat
Sylveon's Attack is too low to make use of any physical moves. It also learns decent coverage moves in Shadow Ball and Psyshock. Sylveon is also only good with Hyper Voice + Pixilate. To even have access to this combo, an Anticipation Eevee with Hyper Voice must be transferred from Black 2/White 2:
-Wish
-Protect
-Hyper Voice
-Heal Bell/Calm Mind
Nature: Bold
EVs: 252 HP/244 Def/12 Spe
Item: Leftovers
Ability: Pixilate
or
-Hyper Voice
-Shadow Ball
-Psyshock
-Hidden Power (Ground)/Baton Pass/Echoed Voice
Nature: Modest
EVs: 244 HP/252 SAtk/12 Spe
Item: Choice Specs
Ability: Pixilate
Charizard X should only use physical moves to take better advantage of its Tough Claws ability. Charizard actually is viable in OU because of the unpredictability of having 2 Mega Evolutions:
-Dragon Dance
-Flare Blitz/Fire Punch
-Dragon Claw/Outrage
-Earthquake/Roost
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Charizardite X
Ability: Blaze
or
-Dragon Dance
-Dragon Claw
-Fire Punch/Earthquake
-Roost
Nature: Adamant
EVs: 144 HP/244 Atk/120 Spe
Item: Charizardite X
Ability: Blaze
or
-Fire Punch/Earthquake
-Dragon Claw
-Will-o-Wisp
-Roost
Nature: Careful
EVs: 248 HP/148 SDef/112 Spe
Item: Charizardite X
Ability: Blaze
Since you already have a Mega Pokemon in Absol, Dragonite makes a fine substitute:
-Dragon Dance
-Dragon Claw/Outrage
-Extreme Speed
-Earthquake
Nature: Adamant
EVs: 24 Def/252 Atk/232 Spe
Item: Lum Berry/Weakness Policy
Ability: Multiscale
You can only Mega Evolve 1 Pokemon per battle, so Lucario will have to make do without Lucarionite:
-Swords Dance
-Close Combat
-Iron Tail/Ice Punch/Earthquake
-Extreme Speed
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
Giratina is an Uber Pokemon, meaning that it's not allowed in an OU team. Ghost Pokemon that are allowed, however, are Gengar and Aegislash:
Gengar:
-Substitute
-Shadow Ball
-Will-o-Wisp
-Taunt/Sludge Bomb/Focus Blast/Sludge Wave/Disable
Nature: Timid
EVs: 28 HP/148 SAtk/80 SDef/252 Spe
Item: Black Sludge
or
-Shadow Ball
-Sludge Bomb/Sludge Wave
-Focus Blast/Thunderbolt
-Destiny Bond
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb
Aegislash:
-Shadow Ball
-Shadow Sneak
-Pursuit/Iron Head/Sacred Sword
-King's Shield
Nature: Quiet
EVs: 244 HP/252 SAtk/12 Spe
Item: Leftovers
or
-Substitute
-Toxic
-King's Shield
-Shadow Ball
Nature: Modest
EVs: 244 HP/252 SAtk/12 Spe
Item: Leftovers
or
-Shadow Ball
-Shadow Sneak
-Flash Cannon/Sacred Sword
-Sacred Sword/Pursuit
Nature: Hasty/Naive
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb/Spooky Plate/Spell Tag
or
-Swords Dance
-Shadow Sneak
-Iron Head/Shadow Claw
-Sacred Sword/Head Smash
Nature: Adamant
EVs: 252 Atk/4 SDef/252 Spe
Item: Life Orb
Regarding your other options, Lapras and Empoleon aren't viable. The former's defensive typing is bad, while the latter doesn't have much that it can offer over bulky Waters besides Defog support (and even then, Crobat, Zapdos, Mandibuzz and
[email protected] do Defog support better, which this team needs if Mega Absol does down).
I don't believe Luxray is viable either. It's slow and not that bulky. Ampharos may be slower, but it has the bulk to take hits. I could suggest Mega Ampharos, or Mega Gardevoir, but you already have Charizard as your Mega. Also, Mewtwo is an Uber, making it not allowed in OU teams.