Don't even think about Delphox. Magician is a terrible ability because a lot of Pokemon rely on their items to function, making going itemless an invalid option. Defense Deoxys is better at using Red Card because of its better overall bulk. Delphox is also outclassed by other Fire Pokemon such as Charizard, Talonflame, Infernape, Heatran, Volcarona and even Victini and Chandelure. These Pokemon have better move pools than Delphox does. Delphox can't even damage Hydreigon or Houndoom (occasional OU sights) neutrally or super effectively outside of Hidden Power:
Charizard:
-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
or
-Fire Blast
-Solar Beam
-Focus Blast
-Dragon Pulse/Roost
Nature: Timid/Modest
EVs: 4 Def/252 SAtk/252 Spe
Item: Charizardite Y
Infernape:
-Fire Blast/Flare Blitz
-Close Combat
-Thunder Punch
-Grass Knot/Mach Punch/Hidden Power (Ice)
Nature: Hasty/Naive
EVs: 168 Atk/88 SAtk/252 Spe
Item: Life Orb/Expert Belt
Talonflame:
-Swords Dance
-Flare Blitz
-Brave Bird
-Roost
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Sharp Beak/Sky Plate
Ability: Gale Wings
or
-Flare Blitz
-Brave Bird
-U-turn
-Tailwind/Roost
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Band/Sharp Beak/Sky Plate
Ability: Gale Wings
or
-Bulk Up
-Brave Bird
-Taunt/Will-o-Wisp
-Roost
Nature: Careful
EVs: 248 HP/8 Def/252 SDef
Item: Leftovers
Ability: Gale Wings
Heatran:
-Lave Plume
-Protect
-Taunt/Roar/Stealth Rock
-Toxic/Will-o-Wisp
Nature: Calm
EVs: 248 HP/220 SDef/20 Spe
Item: Leftovers
or
-Fire Blast/Overheat
-Flash Cannon
-Earth Power/Stone Edge
-Flamethrower/Fire Blast/Hidden Power (Ice)
Nature: Modest
EVs: 252 SAtk/4 SDef/252 Spe
Item: Choice Specs/Choice Scarf
Volcarona:
-Quiver Dance
-Fiery Dance
-Bug Buzz
-Roost
Nature: Bold
EVs: 248 HP/192 Def/68 Spe
Item: Leftovers
Ability: Flame Body
-Fire Blast
-Shadow Ball
-Energy Ball
-Memento/Trick/Hidden Power (Ground)
Nature: Timid/Modest
EVs: 4 Def/252 SAtk/252 Spe
Item: Choice Scarf/Choice Specs
Ability: Flash Fire/Infiltrator
or
-Substitute
-Pain Split
-Fire Blast
-Shadow Ball
Nature: Timid/Modest
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb/Leftovers
Ability: Flash Fire
Victini:
-V-Create
-Bolt Strike
-Brick Break
-U-turn
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Band
or
-Blue Flare
-Thunder
-Focus Blast
-Glaciate
Nature: Modest/Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb/Expert Belt
or
-V-Create
-Bolt Strike/Trick
-Zen Headbutt/Final Gambit
-U-turn
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf
Most Fire Pokemon are weak to Stealth Rock, so Defog/Rapid Spin support is necessary. Mandibuzz, Excadrill, Starmie, Mega Blastoise or Lati@s can do this for you:
Mandibuzz:
-Foul Play
-Defog
-Roost
-Knock Off/Toxic/Whirlwind
Nature: Impish/Bold
EVs: 248 HP/252 Def/8 SDef
Item: Leftovers
Ability: Overcoat
Skarmory:
-Whirlwind
-Roost
-Spikes/Stealth Rock/Defog
-Brave Bird/Defog
Nature: Impish/Bold
EVs: 252 HP/252 Def/4 SDef or 252 HP/16 Def/240 SDef
Item: Leftovers/Rocky Helmet
Ability: Sturdy
Excadrill:
-Earthquake
-Iron Head
-Rapid Spin
-Rock Slide/Stealth Rock
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Leftovers/Air Balloon
Ability: Mold Breaker
or
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe (Choice Scarf) or 120 HP/136 Atk/252 SDef (Assault Vest; Adamant only)
Item: Choice Scarf/Assault Vest
Ability: Mold Breaker
Starmie:
-Hydro Pump/Surf
-Ice Beam
-Thunderbolt/Psyshock
-Rapid Spin
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb
Ability: Analytic/Natural Cure
Mega Blastoise:
-Water Pulse/Scald
-Dark Pulse
-Aura Sphere/Ice Beam
-Rapid Spin
Nature: Modest
EVs: 252 HP/252 SAtk/4 SDef
Item: Blastoisinite
Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Healing Wish
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Latios:
-Draco Meteor
-Psyshock
-Thunderbolt/Hidden Power (Fighting)/Earthquake
-Roost/Recover/Defog
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb