This has to be done in NG+ , first opportunity is obviously P.A.X I didn't like this weapon and even upgrading it to mk VI it did poor damage as I'd not used single rigged so low proficiency.
Use what ever weapon you favour to wear it down then swap to the P.A.X for the killing blow , though I'd recommend swapping sooner like for the last 1/4 of health in case you get carried away