Here is the basic information of the monster I would like to create and put in my campaign and build an encounter around, but I'm not sure how to homebrew this within the current fields provided to homebrew a monster. Any help or suggestions would be greatly appreciated.

Spellbook Golem

Large construct, neutral

Armor Class 17

Hit Points special, see Shells

Speed 30ft levitate.

STR         DEX         CON         INT         WIS      CHA 6 (-2) 20 (+5) 10 (0) 20 (+5) 1 (-5) 1 (-5)

Condition Immunities Blind, deaf, frightened, grappled, poisoned, prone, stunned

Saves Dex +9, Int +9 Damage Vulnerability Fire

Damage Resistances Bludgeoning, piercing, and slashing

Challenge 15 ish

Shells. The Spellbook Golem is made of 8 shells. Each has its own initiative, action, bonus action, and reaction. All move together on the 1st shell's turn. Each has 30 HP. Damage done to one does not carry over to another shell. As a reaction, one shell can move in the way of an area of affect spell and fail the save to take the damage, shielding the other shells. When damage is done to the Spellbook Golem, roll a 1d8 to determine which shell is targeted. A creature can take disadvantage on the attack roll to target a specific shell.

Flaming Shell Explosion. When a shell is reduced to 0 HP by fire damage, it explodes outwards. All enemies within 15 ft of the  Spellbook Golem make a Dex save DC 17, taking 4d6 fire damage on a fail or half on a success.

Counterspell. Each shell may cast Counterspell once per day as a reaction.

i suggest using mythic actions these actions trigger automaticaly when the user reaches a certain amount of hp they then regain some hp and gain the benifits of the mythic action (in this case causing an explosion ) 

ogre

I hear they have layers