I just did a difficulty 6 rocknose purge test next to the sinkhole with the following:
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8x8 floor plan of T3 foundations, all foundations surrounded by T3 fence foundations in grid pattern.
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20 Lians, 5 per side, one foundation length away from the structure. Lian because he’s a guaranteed spawn, and he is completely proficient in using a mace. Meaning he will complete mace combos ALL the time instead of leaving them hanging occasionally. 2 greater wheels will process 16 lians for 10 hours and 20 minutes with a T4 taskmaster.
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No Lians are set to Follow, so no player buffs.
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Each Lian armed with an admin-spawned Reinforced Stool(the joke mace from UC scrolls), no fittings used. This is to simulate the average mass-produced blacksmith-crafted high-end mace that has some sort of damage kit.
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Each Lian is wearing admin-spawned Epic Slaveforged Guardian(they don’t benefit from the +stamina). This is to simulate the average endgame heavy armor set that may or may not have useful boosts for the thrall, and may be have been looted or crafted by a non-top-tier armorer. No fittings were used.
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All 20 Lians are admin’d to level 20. Perhaps this is the most difficult part to achieve organically, as it is a significant time investment for regular play. The Lians are, on average, in the 3600 HP ballpark. One was as low as 2900 HP and another was as high as 4100 HP.
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Probably the key factor that helped was food. All Lians were given a full stack of gruel. So 2000 gruel for 1000 grass and 200 seeds. Additionally, each Lian was given the pork buff, so 20 Cooked Pork Rinds. Not only that, each Lian was given the Salted Pork strength buff, where Salted Pork is placed in their inventory, and manually used. Therefore, +25% damage combined with another +15% damage buff.
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Finally, no player involvement outside of spectating the fight.
First Run
When Rockslide arrived, only half the roster engaged. The collision and crowding would’ve made any more thralls joining the fray a futile effort and take AoE hits without dishing any in return anyway.
After Rockslide died, about 5 Lians were found to be below half health. This did not include the one with only 2900 HP. This is plausible, as about 5 thralls can squeeze into the spot where Rockslide stands, thus take AoE damage about 5 to 6 times as opposed to the edge where the other Lians get hit once or twice.
1 foundation was destroyed, no fence foundations were destroyed.
Second Run
Only half the roster engaged once again, upon Rockslide’s arrival. This time, the two pork buffs had long expired. The end result is similar but 4 Lians were badly injured (visible limping), including the one that had 4100 HP. 1 Lian was killed. 
The one with 2900 HP had more than half HP remaining (might’ve been due to rapid healing from gruel).
No structural repairs were made from the first run, but no additional building pieces were destroyed from the second run. Both encounters with Rockslide started and concluded at the thralls’ standing spot 1 block away from the structure, so structural damage is guaranteed.
No additional food was supplied to the Lians beyond the initial pre-test provisions. One Lian was down to 7 gruel from a stack of a hundred after completely healing through 2 purges.
Impressions
Take the following with the entire Shattered Springs worth of salt:
T4 casualties are seemingly more likely in 3.0 despite decent equipment. How much more likely depends on a laundry list of factors probably. Getting ground to dust by Rockslide, or chopped up by obsidian weaponry? Min-maxing thrall equipment? Good or bad thrall perks? Which thrall is taking the most heat? Any buffs? Will their healing food last? Are you desperate enough to finally use healing arrows? Did the thrall touch a piece of scenery and lose all their situational awareness while taking a beating? Will you then be the hero Funcom said you wanted to be, and swoop in with your shield? I don’t, and probably won’t know enough to say. 
It’s a burden, definitely, to take hours waiting on the wheel, and then farm striped hyenas with them in tow. Perhaps this is to goad us into reserving a portion of our follower capacity for some expendable T3s. They only take 2 hours and 40 minutes for a T4 taskmaster to bake after all. Of course, there’s the matter of getting them some levels.
Or perhaps, this is the time to stock up on Tarmans. He’s a guaranteed spawn, has the same mace proficiency as Lian, and can rack up quite the hit points. Basically a post-nerf dollar-value Teimos , with better mace skill. The kicker in this is that Tarman, like Teimos, is a fast leveller. He’ll be 20 a lot more quickly, and like most thralls moving forward, HP is king more than ever. Have your chaddiest Tarmans tank for your precious Dalinsias or something. Maybe.
I’m not much of a builder, but maybe the builders out there already have their new 3.0 foolproof killzone funnels already in the works. My experiment was a very rudimentary exercise at letting 
hit the blower. I didn’t even use archers to contribute to the DPS from vantage points. With player involvement though, like intercepting incoming waves, or baiting the purge boss to a far away mob of thralls, the situation can and should only improve.
Will any of the above save your settlement from two waves of Sobek Cultists that have more HP each than your T4s? I don’t know. I’ll admit I’m afraid to try, even in admin, for fear of what the result might be. 
Just as much as when it’s a Berserker Purge, and there’s a Purge Confessor on the way. Will my guards survive with only a truncheon? At least with rocknoses, it’s a kill mission.
TL;DR
Probably a good idea to stock up, follower limit permitting, on Black Hand T4s (and even T3s) because they’re the easiest to reach 20 with a sizeable health pool. Also, you train them two at a time maybe. Use them as meat shields for your other, more precious thralls during a purge or something.
That’s it for now. Back to testlive. 
Edit: Manually using the Salted Pork only grants +15% strength damage, like most other foods.