Choice Scarf/Band > Earth Plate on Garchomp. It could use the extra power or Speed. Also consider Dragon Claw over Fire Fang or Stone Edge for a more reliable Dragon STAB.
Tentacruel lost its luster in Gen VI because it can't really deal with Aegislash well. Given the team's relatively offensive nature, Toxic Spikes isn't really going to help. So Starmie is a better alternative for Rapid Spinning:
-Hydro Pump/Surf
-Psyshock/Thunderbolt
-Ice Beam
-Rapid Spin
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb/Expert Belt
Ability: Natural Cure/Analytic
Clefable should be Bold with 252 HP/252 Def/4 SDef.
Charizard Y's last 4 EVs should be put in either defensive stat; putting them in its HP makes it even, making Charizard fall to 2 Stealth Rock switch-ins.
For a tank, try out Aegislash. Though it's bulky, it can turn its huge defenses to its own huge offenses with Stance Change. Clefable provides Wish support for it to make up for its lack of reliable recovery:
-Shadow Ball
-Shadow Sneak/Sacred Sword
-Iron Head/Sacred Sword/Pursuit
-King's Shield
Nature: Quiet
EVs: 244 HP/12 Def/252 SAtk
Item: Leftovers/Spooky Plate/Spell Tag/Air Balloon
or
-Substitute
-Toxic
-Shadow Ball
-King's Shield
Nature: Modest
EVs: 244 HP/12 Def/252 SAtk
Item: Leftovers
or
-Shadow Ball
-Shadow Sneak
-Flash Cannon/Sacred Sword
-Pursuit/Sacred Sword
Nature: Hasty/Naive
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb
or
-Swords Dance
-Shadow Sneak
-Shadow Claw/Iron Head
-Sacred Sword/Head Smash
Nature: Adamant
EVs: 252 Atk/4 SDef/252 Spe
Item: Life Orb
In your last slot, try out something like Amoonguss, Ferrothorn or Tangrowth to work well with the Water and Fire Pokemon you currently have:
Amoonguss:
-Spore
-Giga Drain
-Sludge Bomb/Hidden Power (Fire)/Foul Play
-Stun Spore/Clear Smog
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Black Sludge
Ability: Regenerator
Tangrowth:
-Leaf Storm/Giga Drain
-Knock Off
-Earthquake
-Hidden Power (Ice/Fire)/Rock Slide
Nature: Sassy
EVs: 252 HP/4 Def/252 SDef
Item: Assault Vest
Ability: Regenerator
or
-Giga Drain/Leaf Storm
-Knock Off
-Sleep Powder/Earthquake
-Hidden Power (Ice/Fire)
Nature: Relaxed
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Regenerator
Ferrothorn:
-Leech Seed
-Protect/Thunder Wave
-Gyro Ball/Power Whip
-Stealth Rock/Spikes/Gyro Ball
Nature: Relaxed
EVs: 252 HP/216 Def/40 SDef
Item: Leftovers/Rocky Helmet
Ability: Iron Barbs
or
-Stealth Rock/Spikes
-Leech Seed
-Gyro Ball/Thunder Wave
-Power Whip/Protect
Nature: Relaxed
EVs: 252 HP/216 Def/40 SDef
Item: Leftovers/Rocky Helmet
Ability: Iron Barbs
