Burning dead skellingtons are fire immune, the sorc has a sunder, and does about 1/2 the dps to immunes
Understood, my bad. I did a lot more testing vs p8 immune heralds, and came up with the conclusion that... nova is completely useless against (1) immune herald packs, and (2) high regen tanky herald packs like Urdars. So I just made the merc do all the work for me, and gave support. This strat works for literally any sorc build with 1 point in frost nova and marrowwalks on. Max warmth + max telekinesis + decent ES make it comfortable. Merc can be any a2 or a5 hire.
In my testing on the PTR, heralds don't regen their life as quickly as the pack, weirdly enough. This is what allows the merc to kill them with crushing blow over time without the need for open wounds, poison damage, prevent monster heal from player, etc.
Merc needs cb, ias, cbf, fade, and life leech. Decrepify must be up continuously from Reaper's Toll / Void / Lawbringer for breaking phys immunity/resist for crushing blow, lowering herald damage, and slow. Support sorc must drop frequent sundered frost novas to keep the pack maximally slowed, and bone prison from marrowwalks is imperative for kiting herald away from the pack. Telekinesis can be used sometimes. In tight spaces, play around with closing/opening doors. Abyss from void can be used to keep monsters perma-stunned in certain contexts. Battle cry from CTA / life tap from marrowwalks should never be cast because they override decrep which is the #1 thing keeping the merc alive. Juvs need to be used sometimes, no way around it with this method.
Demonstration on a p8 t5 immune herald, dying in under a minute: