Is this Library-encounter balanced?

Question

I'm about to run a Wizard tower. I really wanted to run a Living Spellbook from Redditor /u/StoneStrix but had to scale it down. Since low-level spellcasters are kind of hard to find, I thought an Elemental could be appropriate as a baseline for this monster.

It will be the final encounter of a one-shot for four 6th-level players (not an optimal party, no power-gamers). The party members are a barbarian (Ancestral Guardian XGE), a fighter (Battlemaster), an artificer (Gunsmith), and a Storm sorcerer. The fighter and the barbarian have +1 weapons. We homebrewed some healing into the Storm sorcerer, he knows healing spirit.

The encounter will contain: 1 Living Spellbook + 1-2 Swarms of Flying Books.

Living Spellbook

Medium construct, unaligned

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Armor Class 14 (17 with Mage Armor)
Hit Points 90
Speed 30 ft., fly 30 ft.

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\begin{array}{cccccc} STR & DEX & CON & INT & WIS & CHA \\ 12 (+1)&18 (+4)&14 (+2)&14 (+2)&10 (+0)&1 (-5) \end{array}

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Damage Immunities poison, psychic
Damage Resistances bludgeoning, piercing, and slashing damage from nonmagical weapons
Damage Vulnerabilities fire
Condition Immunities charmed, exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses blindsight 120 ft., passive Perception 10
Languages -
Challenge 5

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Immutable Form. The spellbook is immune to any spell or spell effect that would alter its form.
Magic Resistance. The spellbook has advantage on saving throws against spells and other magical effects.
Mage Armor. The Spellbook has a permanent Mage Armor on it, which cannot be recast.

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Actions

Multiattack. The Living Spellbook makes two Paper cut attacks.
Paper Cut. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft., or range 30/60 ft., one creature. Hit: 13 (2d8+4) slashing damage.
Arcane Explosion (Recharge 4-6). Each creature within 10 ft. of the Spellbook must make a DC 13 Strength saving throw. On a failure, a target takes 15 (3d8 + 2) bludgeoning damage and is pushed 10 feet away from the Spellbook and knocked prone. If the saving throw is successful, the target takes half the bludgeoning damage and isn't flung away or knocked prone.

Swarm of Flying Books

Medium swarm of tiny monstrosities, unaligned

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Armor Class 13
Hit Points 28
Speed fly 30 ft.

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\begin{array}{cccccc} STR & DEX & CON & INT & WIS & CHA \\ 13 (+1)&16 (+3)&9 (-1)&1 (-5)&7 (-2)&2 (-4) \end{array}

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Damage Resistances bludgeoning, piercing, and slashing damage from nonmagical weapons
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned
Senses darkvision 60 ft., passive Perception 8
Languages -
Challenge 1 (200 XP)

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Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny book.The swarm can't regain hit points or gain temporary hit points.

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Actions

Bite. Melee Weapon Attack: +5 to hit, one creature. Hit: 14 (4d6) piercing damage Or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer.

Answer 1

I didn't do any math, just going off of experience building quite a lot of encounters here. If an exact calculation is what you want, leave a comment and I'll delete this answer.

Coinflip encounter

This encounter will come down to the initiatives rolled. With 13 AC and 28 hit points, neither of the swarms should live more than one attacking round from the party at level 6, even taking into account resistances. They should however take up approximately that round of attack by the players.

Which leaves the Living Spellbook, which gets a practical one round of free attacks because the players will be dealing with the swarms. If they're careless or unlucky (aka, you felt like giving them a hard time), the book will use two multiattacks on the sorcerer or artificer, who should go down fast. At that point, the combat depends on whether they were able to dispel the mage armor. If the swarms got off an attack before dying, I'd say those chances are low. Otherwise, they are high and the fight should be a breeze for the barbarian and fighter (the arcane explosion should do approximately zero against them with a Strength saving throw).

So the 2 scenarios I see happening are:

Swarms have relatively high initiative

• The swarms attack the artificer and sorcerer for about 50% of their HP
• The swarms die
• The Living Spellbook effectively takes out the artificer and sorcerer out of the fight
• Mage armor isn't dispelled, your barbarian and fighter have a hard time, but should barely win out before any deadly casualties. Bad dice rolls for the player result in a loss

Swarms have relatively low initiative

• Swarms die immediately
• Sourcerer dispels mage armor on the Living Spellbook
• Living spellbook becomes a glass cannon with 14 AC, which both the barbarian and fighter easily break
• Fight is a breeze, biggest risk is a small paper cut by one of the twirling paper sheets