Cyrus' Boss was kind of challenging but not too difficult for me, since I was constantly inflicting Poison on the Boss; the best part was that the Boss died from the poison, which made me laugh so hard from it. The boss can summon skeletons which protect him from his weapon/magic weaknesses, so you have to take out the skelly bones first if you wanna do anything to the boss.
For those curious, I used Cyrus[1] (Lv.23), Therion (Lv.21), Alfyn (Lv.20) & Ophilia[2] (Lv.26).
[1] - Cyrus is mandatory for his Chapter.
[2] - Ophilia cannot be changed out of the party.
As for the Chapter itself, Cyrus had to visit one of his old colleagues from the Academy for info on the Tome "From The Depths Of Hell" and where he could find it as it has been missing from the Academy Library for 15 years. In turn, he'll help his colleague out with a strange occurance where citizens have been disappearing.
Finding out that the citizens of the mining town were kidnapped into the sewers of the town. Where they had their blood drained to make crytalized blood shards by someone named Gerion. Upon defeating him and resucing some of the captive citizens, Cyrus then gets a new lead and heads east to find the book once again.