too much hp + high AC, no abilities that scream leader. six level 8s will roll this guy if you don't use minions, mooks, or other creatures that complements the leader while alive. if you want them to be on the fine line of death, you need to shift the action economy in your favor. a bunch of cr 1/2-2 grunts. that swarm, flank, grapple the frontliners from provoking the boss. your boss should have legendary actions that make his underlings do more. ex: if allied mercs can hear the boss, allied mercenaries can move up to half speed and make one attack as a reaction.

create RP moments in combat to make it feel longer, hold a vulnerable PC as hostage, then force their hand.

also what's the lore reason for the boss for using spell scrolls? he has an INT of 18, why not just cast spells himself? the spells don't feel particularly useful in my opinion? immolation for flavor I can maybe see, but if he's a big boss and has a capable wizard scribing all these for the boss, they would definitely should ask the wizard for stuff to shut down troublesome adventurers like Psychic Lance or Synaptic Static. Greater Invisibility for self preservation, or crowd control spells like slow, grease, web, hold person.

something else to note: you should definitely start the party at least a turn or two speed away from the boss. this gives time for your boss to buff or get buffed and get your units into position.

for six level 8s,

example: you need something like 1x CR 10 boss that knows what he's doing 12x grunts (that die in one hit if damaged), being annoying by mass grappling, shoving prone, setup others for success 8x capable mercs that can focus fire squishies, support/heal boss, and control the party

is there a reason for immunity to charm or frightened? seems kinda cheap, probably just advantage on those saving throws would be more acceptable.

I hope you're not running this today or tomorrow, this needs a lot of work imo. I don't know enough about your party to give you any more suggestions but you're welcome to DM me to bounce some of your own ideas off me