In the long run, Assault Vest is the best item for Conkeldurr; even with Drain Punch, it tends to get worn down by burn or poison pretty easily. The point of Guts is to punish opponents who recklessly throw around status moves:
-Drain Punch
-Mach Punch
-Ice Punch/Knock Off
-Poison Jab/Stone Edge
Nature: Adamant
EVs: 252 Atk/236 SDef/20 Spe
Item: Assault Vest
Ability: Guts
Reflect Type is the preferred option on bulky Starmie so that it isn't Pursuit bait for Tyranitar or Bisharp:
-Scald
-Reflect Type
-Rapid Spin
-Recover
Nature: Timid
EVs: 248 HP/20 Def/16 SDef/224 Spe
Item: Leftovers
Ability: Natural Cure
Air Slash is unnecessary on Charizard Y because sun-boosted Fire Blasts outdamage 2x super effective Air Slashes:
252 SpA Mega Charizard Y Fire Blast vs. 252 HP / 0 SpD Thick Fat Mega Venusaur in Sun: 270-318 (74.1 - 87.3%) -- guaranteed 2HKO
252 SpA Mega Charizard Y Air Slash vs. 252 HP / 0 SpD Mega Venusaur: 246-290 (67.5 - 79.6%) -- guaranteed 2HKO
252 SpA Mega Charizard Y Fire Blast vs. 0 HP / 236 SpD Assault Vest Conkeldurr in Sun: 220-261 (62.6 - 74.3%) -- guaranteed 2HKO
252 SpA Mega Charizard Y Air Slash vs. 0 HP / 236 SpD Assault Vest Conkeldurr: 200-236 (56.9 - 67.2%) -- guaranteed 2HKO
And speaking of Fire Blast, it's preferred over Heat Wave for its better power. Its accuracy is still acceptable. The only time you'd use Heat Wave is in doubles, when it hits both opponents, but this isn't a doubles team:
-Fire Blast/Flamethrower
-Solar Beam
-Focus Blast
-Roost
Nature: Timid/Modest
EVs: 40 HP/252 SAtk/216 Spe
Item: Charizardite Y
No matter how tempting it is to run Chlorophyll Pokemon alongside Charizard Y, it's not worth it anymore because Drought now lasts for 5 turns (8 with Heat Rock attached; Ninetales or Groudon only). The point of Charizard's Drought is to make its Fire moves more dangerous and making Solar Beam a 1-turn attack. In other words, Charizard is generally the only Pokemon that should benefit from the sun it brings. This team needs a Stealth Rock setter; Garchomp, Therian Landorus, Tyranitar or Hippowdon can do this for you:
Garchomp:
-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/Rocky Helmet
Therian Landorus:
-Stealth Rock
-Earthquake
-U-turn
-Stone Edge/Rock Slide/Knock Off
Nature: Impish
EVs: 152 HP/252 Def/104 Spe
Item: Leftovers
Tyranitar:
-Stealth Rock
-Crunch
-Stone Edge/Ice Beam
-Earthquake/Fire Blast
Nature: Impish/Relaxed
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Sand Stream
Hippowdon:
-Stealth Rock
-Earthquake
-Rock Slide/Toxic/Whirlwind
-Slack Off
Nature: Impish
EVs: 252 HP/72 Def/184 SDef
Item: Leftovers
Why do you have Acrobatics on Talonflame when it already has a strong STAB in Brave Bird? While it's true that Acrobatics gets powered up when Talonflame loses Choice Band (to Knock Off, usually, but Talonflame is unlikely to live a boosted Knock Off), it's an inconsistent option. All-out offensive Talonflame should look something like this:
-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
While it's true that Drought cancels out any Water weaknesses this team may have, it may be best to put in something that checks/counters Water Pokemon. Since I removed Venusaur, you'll have to squeeze in stuff like Amoonguss or
[email protected] (either one should go over Metagross; it's not that good now because of its vulnerability to Dark and Ghost, types that it used to handle rather well with neutrality):
Amoonguss:
-Spore
-Giga Drain
-Foul Play/Hidden Power (Fire)
-Sludge Bomb/Clear Smog
Nature: Bold
EVs: 252 HP/136 Def/120 SDef
Item: Black Sludge
Ability: Regenerator
Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Healing Wish/Roost/Recover
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb/Leftovers
Latios:
-Draco Meteor
-Psyshock
-Surf/Hidden Power (Fire)/Thunderbolt
-Defog
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb