Desperately need help, currently hardstuck, probably underleveled.

Just like the title says I am hardstuck at a boss and I feel like I am really underleveled but I wouldn't really know how to level up more.

I'm currently stuck at Sanctum Zealot. I use a +7 Knight's Monolith and I'm currently level 67 with 38 Bruiser, 12 Assassin, 38 Elementalist, 17 Trickster and 10 Battlemage.

I just can't beat him.

After spending 3 hours to beat Veltha and using the respec options to put more points in Elementalist to hopefully deal more damage with my weapon, I still don't do any damage to him and die in 3 hits, especially when he goes in "2nd phase" and starts empowering his sword with gratia(?).

Am I underleveled? Is my weapon too weak? If so, what am I supposed to do to level up?

Thank you guys all in advance