For your blastoise, try using a 252/252/0/0/4/0 or so. Definetely have king's shield on Aegislash. Aegislash can still be incredibly powerful with a defensive EV build, such as 252/4/0/0/252/0 Make sure you have 0 EVs on speed so you can attack after other pokemon, allowing you to take hits in shield form.
You don't understand how Stance Change works. It simply lets Aegislash switch formes depending on how it attacks; it doesn't switch the actual base stats, as both of its formes share the same EVs. Look at
Stance Change's article, then
the base stats of Aegislash's formes.
Ex: If you invest in Attack, the Attack EVs are carried over from Shield forme to Blade forme. At level 100, Shield forme Aegislash's base offenses are at 218 fully invested (base 50 Attack). That's normally weak, but when Stance Change activates, it changes to a Blade forme with base 150 Attack, which has 438 Attack.
Not all Aegislash run King's Shield because using it can cause it to become setup bait. It should only be used if you can predict an offensive move.
Anyway, Mega Blastoise is too slow to utilize Water Spout effectively; as it has a tendency to take hits before it can dish them out, which weakens Water Spout. Iron Defense does Mega Blastoise no favors either, as it becomes a critical hit magnet. Your EV spread is inefficient. It doesn't give Megatoise as much power as it needs and gives it inefficient bulk:
-Water Pulse/Scald/Hydro Pump
-Dark Pulse
-Aura Sphere/Ice Beam
-Rapid Spin
Nature: Modest
EVs: 252 HP/252 SAtk/4 SDef
Item: Blastoisinite
Aegislash's mixed attacking sets should look something like either of these:
-Shadow Ball
-Shadow Sneak/Sacred Sword
-Sacred Sword/Iron Head/Pursuit
-King's Shield
Nature: Quiet
EVs: 244 HP/12 Def/252 SAtk
Item: Leftovers/Spooky Plate/Spell Tag
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
While Mega Blastoise and Aegislash are bulky, they can be worn down easily because they lack reliable recovery. Sylveon or Clefable can provide Wish support for both:
Sylveon:
-Wish
-Protect
-Hyper Voice
-Heal Bell/Calm Mind
Nature: Bold
EVs: 252 HP/216 Def/40 SDef
Item: Leftovers
Ability: Pixilate
Clefable:
-Wish
-Protect
-Moonblast
-Heal Bell/Thunder Wave/Stealth Rock
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Unaware/Magic Guard
Other than Mega Blastoise, you need a way to deal with Bisharp. Conkeldurr or Garchomp can do either of those things:
-Swords Dance
-Outrage/Dragon Claw
-Earthquake
-Stone Edge/Fire Fang
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Lum Berry
or
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Focus Sash/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
or
-Stealth Rock
-Dragon Tail
-Earthquake
-Toxic/Fire Blast
Nature: Impish
EVs: 252 HP/252 Def/4 SDef
Item: Rocky Helmet
Ability: Rough Skin
Conkeldurr:
-Drain Punch
-Mach Punch
-Knock Off
-Ice Punch/Stone Edge
Nature: Adamant
EVs: 252 Atk/236 SDef/20 Spe
Item: Assault Vest
Ability: Guts
or
-Bulk Up
-Drain Punch
-Mach Punch
-Ice Punch/Stone Edge/Knock Off
Nature: Adamant
EVs: 252 Atk/236 SDef/20 Spe
Item: Leftovers
Ability: Guts
You need an entry hazard setter to help your Pokemon get KOs they normally can't. Ferrothorn or Skarmory provide hazard support:
Ferrothorn:
-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
or
-Leech Seed
-Protect/Thunder Wave
-Gyro Ball/Power Whip
-Stealth Rock/Spikes/Gyro Ball
Nature: Relaxed/Impish
EVs: 252 HP/216 Def/40 SDef
Item: Leftovers/Rocky Helmet
Ability: Iron Barbs
Skarmory:
-Whirlwind
-Roost
-Stealth Rock/Spikes/Brave Bird
-Defog/Brave Bird
Nature: Impish/Bold
EVs: 252 HP/252 Def/4 SDef or 252 HP/16 Def/240 SDef
Item: Leftovers/Rocky Helmet
Ability: Sturdy
Last slot is up to you.