[RPG] Gaining immunity to physical damage

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 sourcerer 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 agains them with a Strength saving throw).

So the 2 scenarios I see happening are:

Swarms have relative high initiative

• The swarms attack the artificer and sourcerer for about 50% of their HP
• The swarms die
• The Living Spellbook effectively takes out the artificer and sourcerer 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 relative 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

Sneak attack damage is magical if the weapon used is magical.

The Sneak Attack feature says that the rogue adds extra damage to the weapon attack.

Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.

This implies that the extra damage is the same type of damage as the weapon. If the rogue's weapon is magical, and the sneak attack damage is added to the weapon's damage, then you would count the total as damage from a magical weapon.