I made some notes with bold.
Greninja is an Uber in OU play. It gained Gunk Shot and Low Kick in OR/AS to better handle its checks/counters, so you should be using those:
-Hydro Pump/Low Kick
-Ice Beam
-Gunk Shot
-Dark Pulse/Grass Knot/Hidden Power (Fire)
Nature: Naive
EVs: 40 Atk/216 SAtk/252 Spe
Item: Life Orb
Ability: Protean
For Garchomp, Dragon Tail should only be used on defensive sets; it won't do much merit on offensive ones, as the negative priority will screw you over so hard:
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Focus Sash/Lum Berry/Rocky Helmet
or
-Swords Dance
-Outrage/Dragon Claw
-Earthquake
-Fire Fang/Stone Edge/Substitute
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Lum Berry
or
-Outrage
-Earthquake
-Stone Edge/Rock Slide
-Fire Blast/Dragon Claw
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Choice Scarf
Mega Aggron should provide Stealth Rock support if you don't have that already:
-Stealth Rock
-Heavy Slam/Iron Head
-Thunder Wave/Avalanche/Ice Punch
-Earthquake/Rock Slide
Nature: Impish
EVs: 252 HP/16 Def/240 SDef
Item: Aggronite
Ability: Sturdy
As rob_thjis already mentioned, there are better Dark Pokemon than regular Absol. However, regular Absol is terrible because of its frailty and subpar Speed. The better Dark Pokemon to use are Weavile and Bisharp:
Weavile:
-Knock Off
-Icicle Crash
-Ice Shard
-Low Kick/Pursuit/Poison Jab
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
Ability: Pressure
Bisharp:
-Sucker Punch
-Iron Head
-Knock Off
-Pursuit/Swords Dance
Nature: Adamant/Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Black Glasses/Dread Plate/Life Orb
Ability: Defiant
or
-Sucker Punch
-Iron Head
-Knock Off
-Pursuit
Nature: Adamant
EVs: 192 HP/220 Atk/96 Spe
Item: Assault Vest
Ability: Defiant
Like Absol, Gardevoir has a subpar vanilla form and is only used for its Mega Evolution. Better Psychics and Fairies to use are Gothitelle (for trapping counters with Shadow Tag), Latios (for more power and Speed), Starmie (to provide Rapid Spin support), Azumarill (for a hard-hitting attacker), Sylveon and Clefable (both to handle Mega Sableye better):
Gothitelle:
-Psychic
-Thunderbolt
-Hidden Power (Ground)/Rest
-Trick
Nature: Modest
EVs: 168 HP/252 SAtk/88 Spe
Item: Choice Specs
Ability: Shadow Tag
Latios:
-Draco Meteor
-Psyshock
-Hidden Power (Fire)/Surf/Earthquake
-Defog/Roost
Nature: Timid/Hasty
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
or
-Draco Meteor
-Psyshock
-Tailwind/Defog/Hidden Power (Fire)
-Trick
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Choice Scarf
Starmie:
-Scald
-Rapid Spin
-Reflect Type
-Recover
Nature: Timid
EVs: 248 HP/16 Def/244 Spe
Item: Leftovers
Ability: Natural Cure
or
-Hydro Pump
-Psyshock/Thunderbolt
-Ice Beam
-Rapid Spin
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb/Leftovers
Ability: Natural Cure
Azumarill:
-Waterfall
-Play Rough
-Aqua Jet
-Knock Off/Superpower
Nature: Adamant
EVs: 164 HP/252 Atk/92 Spe (Choice Band) or 240 HP/252 Atk/16 SDef (Assault Vest)
Item: Choice Band/Assault Vest
Ability: Huge Power
or
-Belly Drum
-Aqua Jet
-Play Rough
-Knock Off
Nature: Adamant
EVs: 92 HP/252 Atk/164 Spe
Item: Sitrus Berry
Ability: Huge Power
Sylveon:
-Wish
-Protect
-Hyper Voice
-Heal Bell/Toxic
Nature: Bold
EVs: 252 HP/220 Def/36 SDef
Item: Leftovers
Ability: Pixilate
or
-Hyper Voice
-Psyshock
-Shadow Ball/Hidden Power (Fire/Ground)
-Baton Pass
Nature: Modest
EVs: 244 HP/252 SAtk/12 Spe
Item: Choice Specs
Ability: Pixilate
Clefable:
-Calm Mind
-Moonblast
-Soft-Boiled/Moonlight
-Flamethrower/Thunder Wave
Nature: Calm
EVs: 252 HP/172 Def/84 SDef
Item: Leftovers
Ability: Magic Guard
or
-Calm Mind
-Moonblast
-Moonlight/Wish
-Heal Bell/Protect
Nature: Bold
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Unaware
or
-Stealth Rock
-Moonblast
-Fire Blast
-Soft-Boiled/Moonlight
Nature: Calm
EVs: 252 HP/184 Def/72 SDef
Item: Leftovers
Ability: Unaware
I'm going to be blunt here, Dusknoir sucks. It has a good move pool, sure, but it lacks a good Ghost STAB to take advantage of. The better defensive Ghosts to turn to are Cofagrigus and Super Size Gourgeist:
Cofagrigus:
-Trick Room
-Nasty Plot
-Shadow Ball
-Will-o-Wisp
Nature: Quiet
EVs: 248 HP/8 Def/252 SAtk
Item: Leftovers
Super Size Gourgeist:
-Will-o-Wisp
-Leech Seed
-Synthesis
-Seed Bomb/Shadow Sneak
Nature: Impish
EVs: 252 HP/160 Def/96 SDef
Item: Leftovers
Ability: Frisk