Fire sunder yes, phys sunder no. Even though you can use multiple non-renewed, bone break is a net negative. In theory it lets amp+sigil apply 5x the effect to phys immunes, a huge boost. In practice, virtually every native phys immune monster has low enough hp and non fire immune that they die in 1 hit to the fire damage component even while still phys immune.
Its absolutely enough damage to run P8 and kill everything including immunes and bosses and tanky packs like frenzytaurs or even t5 heralds. Its about twice the damage per cast of hammers, in a huge instant aoe. I hit around 12.5k combined damage, so up to 25000 hp lost after all those stacked -resists / dr. Skill rotation is just teleport blood boil spam and deathmark as a teleport alternative, and the engorge after you wipe a room. Sigil is almost always overkill, you can use it on stuff like bosses and heralds but almost never on normal packs. And of course, a brief buff with battle command / battle orders / siphon / defiler / tainted / consume. Gets to be muscle memory
I keep deathmark on leftclick to easy aim the demon to clear rooms before I enter them, or pick off a straggler, etc. Absolutely neccessary doing areas with dolls, but still immensely helpful against the stuff that can interrupt you like vipers, charge zombies, suicide minions, etc if you see no reason to face tank frenzytaurs or burning souls. Particularly to keep your merc alive, deathmark is great at putting your demon in front of a herald pack that would kill your merc but not even scratch a demon
I just dont see the need for stormshield. A max blood oath warlock is just so ridiculously tanky I havent died on softcore being incautious in chaos / swampy pit / wsk from level 90 to 96 or so.
I'm running bb, boss killing solo in p5 above where there's no corpses like baal chamber or ancients is an issue though... For baal I have to tp back to heal demons which is a hassle. Any tips?