YAY! I just killed Vhaksiz the Shade in PoN this morning.
The worst part was overcoming his regen. Bane+Sisslak+ panther puppy had him bouncing between 99 and 100%. Ran to icefall and bought Juju (never really needed it before, so sue me). Came back, stacked up Anathema, Bane, Sisslak, and Juju on him, along with panther puppy and slowly took him down over about a 20-25 minute fight.
He slowed alright, didn't hit too hard. Lassitude was enough to keep the pet healed from the minimal rampage, was resistant to poison and disease (thus the curse dots), and he dropped the shoulders I was after for my warrior twink.