As an FYI, Charizard Y teams are very specialized, as while Charizard Y is a strong nuke under the sun, it struggles with some Pokemon, especially the Latis and Chansey. Megazard Y teams need the following:
-Fighting Pokemon that resists Steel for handling Bisharp and Chansey
-bulky Ground Pokemon to deal with Rock Pokemon (Therian Landorus, Hippowdon, Garchomp)
-Rapid Spin/Defog support for getting Stealth Rock off (better ones are Excadrill and
[email protected]) (Therian Landorus, Hippowdon, Garchomp)
-Pursuit user for dealing with opposing
[email protected] (Bisharp, Tyranitar, Scizor)
-Bird spam check/counter to get rid of Talonflame, Staraptor or Mega Pinsir (Wash Rotom, Thundurus, Zapdos)
And judging from the team above, you have most of the criteria (Raikou, Garchomp), even though Donphan is an outclassed Rapid Spinner in standard, as Excadrill has more offensive presence.
Arcanine is outclassed in standard because while it learns good moves, it doesn't have much to set itself apart from other Fire Pokemon like Charizard (which has two Mega Evolutions), Heatran and Talonflame.
Why, oh why, are you using Choice Specs Garchomp? While it learns a decent set of special moves, its average Special Attack makes a full Special set a terrible idea. It should only use its Special Attack if it uses a physically mixed spread:
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Lum Berry/Focus Sash
I'd replace Donphan with Excadrill, as it also packs Rapid Spin to clear away entry hazards while posing more of an offensive threat:
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf/Air Balloon
Ability: Mold Breaker/Sand Rush
or
-Earthquake
-Iron Head
-Rapid Spin
-Toxic/Stealth Rock
Nature: Careful
EVs: 252 HP/4 Def/252 SDef
Item: Leftovers
Ability: Mold Breaker
As for Arcanine and Gliscor, they should be replaced for the Pursuit user and Fighting Pokemon.
Pursuit users:
Bisharp:
-Sucker Punch
-Iron Head
-Knock Off
-Pursuit
Nature: Adamant
EVs: 252 Atk/52 SDef/204 Spe (first three items below) or 192 HP/220 Atk/96 Spe (latter item below)
Item: Life Orb/Dread Plate/Black Glasses/Assault Vest
Ability: Defiant
Tyranitar:
-Stone Edge
-Crunch/Ice Punch
-Pursuit
-Superpower
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe (Choice Scarf) or 100 HP/252 Atk/156 Spe (Choice Band)
Item: Choice Scarf/Choice Band
Ability: Sand Stream
or
-Stealth Rock
-Stone Edge
-Crunch/Ice Beam/Fire Blast
-Pursuit
Nature: Relaxed
EVs: 248 HP/36 Def/224 SDef
Item: Smooth Rock/Leftovers
Ability: Sand Stream
Scizor:
-U-turn
-Bullet Punch
-Superpower
-Pursuit
Nature: Adamant
EVs: 248 HP/252 Atk/8 SDef
Item: Choice Band
Ability: Technician
Fighting Pokemon (that resist Steel):
Infernape:
-Fire Blast/Flare Blitz
-Close Combat
-Mach Punch
-Hidden Power (Ice)/Grass Knot/Thunder Punch
Nature: Naive/Hasty
EVs: 168 Atk/88 SAtk/252 Spe
Item: Life Orb
Ability: Iron Fist
Keldeo:
-Hydro Pump
-Secret Sword
-Scald
-Hidden Power Electric/Bug)/Icy Wind/Taunt
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Choice Specs/Choice Scarf/Life Orb
or
-Substitute
-Calm Mind
-Scald
-Secret Sword
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Leftovers