I may be stepping out on a limb here but my hdin merc has always used EBOTD War Pike, Eth Fort, Guil Helm socketed with 40/15 jewel (40 enhanced dmg 15 ias) or Andy's socketed with 40/15 jewel.
With concentration aura on my merc was hitting close to 10k damage and that was with a random ED % EBOTD. If you want to invest you can get a 400% or even a 415% and to maximize damage. I used this set up when I was running chaos and baals, immunes were always killed quickly and mana was never a problem.
You can put an additional skill point into meditation or redemption (my preference) and switch to that aura to absorb some souls quickly. (Generally between waves) If you don't want to use that skill point you could always carry a phoenix pally shield around and swap to activate redemption to absorb souls, but that can get tedious.
The only use I can see using treachery for would be to buff fade which is nice but I think the higher defense, chilling armor, and enhanced damage is going to make your merc overall more damage dealing to take care of the immunes which seems to be the goal here.
Lmk what you think, I highly suggest trying the EBOTD WP
