Idea for a 3.0 Hierophant

I had an Idea for a build, which combine the +max elemental resistance of saffel's frame and purity auras(fire, ice lightning) and the conviction of power hierophant node to minimise the overall damage taken, in order to create a char which can shrug of most if not all damage sources.

In more detail I want to use firestorm (or any other skill which can deal lots of hits per second) in combination with power charge on crit support, or voll's protector or romira's banquet unique, in order to generate power charges fast which then in turn generate endurance charges to fuel immortal call. This way I could get a lot of immortal call uptime, especially during boss fights.

The second half of this build aims to use the saffel’s frame unique (+4 to all max resistances, which means 16% less elemental damage) in combination with purity auras (they give +4% max resistance at level 20 and +5% at level 23, relatively easy to achieve with a geoffrie’s crest or alphas howl and a empower gem, this can get even stronger with aura nodes) to minimise the elemental taken. The auras can be turned on and off as needed. This also works great against reflect.
I am however not very good at theorycrafting and don’t know the 3.0 skilltree yet. So I need a bit of help for the exact details. Especially since there aren’t many similar builds. There are righteous fire build’s which try to raise max resistance as much as possible so, that’s a start. Conviction of power however seems to be a bit under used this season, maybe because most crit build go for inquisitor.
One last thing is, that I think that at least with fire storm the build would work best with the additional skill effect nodes and no additional area of effect due to how hitboxes and firestorm work.

Thank for your help guys.

P.S tell me when you're not sure how the build is supposed to work.

There are a few things wrong with your line of thought, but it's not a bad idea you got there going.
Firstly why hierophant, it seems very poorly picked other than the crappy endurance charge generation it offers you nothing else. You'd be better of with a guardian or even a champion or juggernaut if the goal is to minimize damage.
Secondly the length that it takes to obtain such resistance caps even with legacy saffel's frame is huge, if your build is only going that path for the resistance caps, you'll end up with no damage or physical mitigation. For instance RF, even though it goes to some lengths to increase fire res cap, it's in order to make the skill fluid and work itself and not only defensives. And since that's the build enabling feature they can then focus on mitigation from physical sources and other things while getting highlife/energy shield (which also increases their dmg). What i'm saying here is that they are able to not build "damage for say" since the damage itself comes from their own defensive mechanics. On your case you would be sacrificing way to much with no end goal other than take less elemental damage leaving you without phys mitigation and more even damage to clear any content whatsoever.

Empower should never, NEVER, be used with auras, it will spike the auras cost stupidly high without need. There are other options such as unset +3 rings, or gear pieces that increase socketed gems and/or socketed aura gems level.

Phox (a well known twitcher/youtuber in the community), during the last months has invested greatly in theorycrafting and testing various things regarding low life RF, on a guardian.
Achieving great defenses, and utterly huge damage numbers, and he goes greatly in depth on his explanations and progresses.
If you check him and see a view videos you might get a diferente grasp on the matter, and understand better how RFs, and support characters are the only ones able to achiev such things since they either do no damage or simply scale damage with defenses both ways.

Moreover in one of his RF videos (can't remember which one exactly) Phox does go over on how to achiev 102% all ele resists therefor being immune to ele damage, and explains all the mechanics involved.

To sum up, your ideia is indeed a thing, it's possible, quite expensive, but also kind of useless since you can't clear any content with it.
However you can go on Path of Building and test things out, of how far you can take the numbers. Gl & Hf.