I have just beaten the boss in the DLC after completing the Palace of Lathyrus. The difficulty of the DLC felt fine at this point, neither too easy nor too hard. The boss itself took me 3 tries. On the first 2 tries I was fighting him alone and it was somewhat difficult to find openings to attack, especially in phase 2 I was only able to hit him once after he jumps, so it was a really tedious fight. For the third try I made sure my companion would survive to be part of the boss fight and suddenly it was super easy.
Used the mask with additional damage via the orbs, defiler's veil for good stamina regen and a maxed out one handed axe + shield. Just stayed defensive until he focused my companion, then hit him until he changed aggro to me, and repeat until he was dead.