>return to town, swap on nightwings, fathom, 20/20 shield, ormus, cold skillers and token
stat into thunderstorm and tele around for 30 min
200 fcr nova dropping a p8 T5 medium tanky non-immune herald in ~60 sec with slowing merc, no infinity, no pots used
If the herald were light immune and high health, it might double or triple the time to kill.
Blackhorn's face on merc has prevent monster heal. Kelpie snare can be used instead of Reaper's Toll. Infinity + cure + prayer merc can work too, but requires applying a sundered frost nova every now and then. If going self-wielding infi + insight on prayer merc, you might need to swap out cure helm for blackhorn's for extra slow and prevent monster heal, and you'll still need to drop sundered frost novas here and there.
Other ideas I toyed with:
decrepify on void rw (useful if merc is not using Reaper's Toll or Kelpie Snare)
abyss skill on void rw (useful for keeping a slow herald away if merc dies)
telekinesis to keep herald just out of reach (unreliable but can work vs slow heralds)
slow missiles on peace rw (useful if herald is a soul or something else that shoots)
doom axe (not great, got more utility out of sundered frost novas)
marrowwalk boots for bone prison & life tap for merc (keeps you + merc alive vs tough heralds)
If the herald has low regen, then 200 fcr nova can bring it down without a merc, just by dropping occasional sundered frost novas, even if the herald is light immune. Using wizzy and maxing warmth means no potting is necessary except for occasional hits to the life pool. The same cannot be said for self-wielding infi, which needs the merc alive for sustain.