https://wiki.starknightessaura.com/en/Quests/reverse-summoning-ritual
Hm, I think there might be a problem here actually. So, one way to defeat Dolus is to make his claims naturally true (making his lies backfire). For example, by turning on Offensive Stance. However, on Story difficulty he's just too weak for that. The reverse holds true on Nightmare.
I will be making a balance change for the next patch to multiply the damage threshold with the difficulty stat modifier. He would then only deal 125 damage on Story mode and if you can raise his natural damage above 125, his book will explode.