Crystal Project: VS Periculum (Hard Mode)

With some good counter play, you can utterly humiliate even this secret boss.
Warlock/Hunter is holding the Hope Cross in case he has to attack through a counter kill during hazard stance.
Assassin/Rogue is using whatever, her shadow cut/sneak attack doesn't trigger counters. The only risk of running into a counter kill is if she has to trick slash a Dark Overture during hazard stance, which didn't occur in this run.
Reaper/Warrior has the Unreactable passive, so no risk of eating a counter kill. I picked Reaper as my tank of choice for this fight, as the max HP increase effect can enable him to survive a high roll from the 100% variance fireblasts. Being able to survive those means that the Rogue doesn't have to waste AP interrupting it. He was also holding a Ribbon to avoid DoTs from that one physical attack and fireblast. He is also wearing the Plate of Whale, although the HP increase effect from that is mainly so he can survive the first fireblast if it rolls high. By the time any further fireblasts come out, he'll have enough max HP increases to survive anything. The max HP increase effects also means that the Regen ticks from my Warlock's buff will heal more over time as well.
My Summoner/Scholar spent most of the fight buffing and healing, throwing out the offensive summons during the Hazard Stance phase. The boss is actually most vulnerable during the phase as long as you have a solid plan to avoid eating a counter kill. Counter Kill is considered a physical attack, so blinding the boss effectively disables the counter until their turn rolls around.
Despite the large moveset that the boss possesses, I've noticed that at least during hard mode, he starts off heavily favoring his magical moveset. Still, he did Pain Spreader right after shifting into Hazard Stance, so you should always be prepared for that possibility regardless.
Granted, the way I play this game means I'm not 100% sure if the boss is just reacting to my particular playing style (because I really have no explanation for why he isn't using his single target physical attacks unless I silence him), and in this run I managed to kill him before he completed a full stance cycle.
Also, this clear is almost 3x faster than my normal mode run. A testament to how a player's tactics can evolve with greater experience over time.

[@cheese0_4]

I've never seen such an effective tanking method! I only really used Reaper during the midgame desert/mountains part of my first playthrough and didn't really touch it again, so it's cool to see optimised usage of it!

[@veinae]

yo bro why no more upload, i like your content :(

[@Alilatias]

Oh, I haven't dabbled with the new chaos mode yet. Work + my attention being dragged away to other games that aren't nearly as interesting to record, ha. I'll probably pick up Crystal Project again in a couple months.

[@LantisTDH]

There's definitely something going on with the AI. I couldn't get him to stop spamming Dark Overture. I took 6 over the course of the fight before falling to a Pain Spreader when he was at red health. Also, Trick Slash is not viable due to rouge never getting a turn between when Perculum starts charging and actually casting Dark Overture.

[@NapkinDictator]

You could try using a weaver in the fight too. Daze and Dust would help tremendously with getting a trick slash in at the right time

[@octaviomoran8475]

Helloo, can you do a guide for Potion Permit? :)