So, I took off all my mitigation CP and took off all armor and passives to land on 0 mitigation.
I went for 0 physical resist to see how much damage heavy attack base damage do, so I could calculate if my mitigation against elite adds seemed sufficient with a mitigation calculator:
https://jscalc.io/embed/SSBseoHKGQmww17t
Mitigating 70% dmg is all fine, if you don't know what numbers you mitigate 70% dmg off "somthing" is kinda a shot in dark.
I got my physical resist to 0 but my spell resistance stayed on 3300 which roughly translated to 34% mitigation (according to calculator) if their attacks also applied spell damage.

The results were interesting to say the least, I didn't know heavy attacks at its base did well over 100.000 damage.
I tested elite adds in 3 vet dlc dungeons so far:
vFalkreath Hold - Dreadhorn Minotaur
Heavy Attack: 120.200
vStonethorn - Werewolf Packleader
Heavy Attack: 141.831 (the 3k spell resist may have influenced this - I think it is closer to 150k)
Also apply a nasty bleed if not blocked
vFrostvault - Coldsnap Ogre
Heavy Attack: 146.356

Assuming I'm not wrong here - which I may be.
A 35k health tank with max ironclad and hardy/elemental defender, 20k resistances, 2x sword and board and dragonknight ironskin passive - while blocking - would have an 83% dmg mitigation
Say that person got hit with a 145.000dmg attack he would still lose 24.650 hp leaving him with around 10k health. Without quick heals we would not survive 2 of these in rapid succesion.
If we don't block but instead apply minor maim and minor protection on top of our resistances - that same attack would hit us for around 37.000 damage, which would unfortunately still one-shot us.
If we got our resistances up to 25k we would survive it with 2k health left assuming we were at full health when we got struck.
Again, my math may be wrong here. I based it off of the calculator mitigation and a flat 145k incoming dmg.
If someone who knows more than me on this would/could add to it that would be awesome.
But yeah, atleast I now got a rough estimate on incoming damage numbers.