skills that work and skills that don't (taking a guess)
From what I can see, the last boss now isn't affected by a lot of skills.
I have personally tested it out in solo mode, each and every one of the skills (when there isn't any health stone).
I've grouped the skills into the ones that damages and ones that doesn't damage the boss (for me).
effective:
shadow dance, leap, hellblades, Unholy force, Unholy blessing (although it is used on you)
Not effective:
fatal burst, curse, dark binding, vermin swarm, fire aura
By examining the different skills I came this prediction. All "physical attacks" work and all "magical attacks" fail to work.
All effective skills are forms of "physical attacks" meaning that you attack the boss directly (including normal attack of course). Hellblade are also a form of melee attack even though its a medium distanced and not close ranged attack. Prove: description- you throw your blades forward in a cone attack.
All non-effective skills are forms of "magical attacks". Obviously you cast a spell on the boss with these three attacks: fatal burst, curse, and dark binding. Fire aura is conquering up fire around your body which is also a form of magical attack. Although the rats bit your opponents, you have to "summon it" which makes it magical too.
