I was still getting one shotted occasionally.
Because armor doesn't work the way you might think it does. It deals best with a lot of small hits, and becomes increasingly worse against big hits, requiring higher and higher amounts (which are essentially unreachable) to mitigate some of the hardest hitting abilities in the game. Against high amounts of physical damage you are thus left with some very weak defensive layers.
-Life regen doesn't protect you against big hits
-Neither does vaal MS (you're better off using a regular molten shell, due to the fact that while it absorbs a smaller amount of damage, its more than twice as effective against any single hit with its 75% mitigation vs vaal MS 35%
-Running divine flesh with only 83% chaosres seems a bit redundant. while it does reduce the damage taken from elemental hits a bit, the investment to get there (as in, the amount of suffixes/passives you spend on getting said chaosres) is not worth the mitigation you gain from it. Unless you're sitting at 85-90%, divine flesh becomes too heavy to invest into for its benefit, and you're most likely losing out on damage which would help you kill threats faster, or other ways of mitigation or avoidance that you could've had access to instead. You will also want some physical as elemental damage taken with it.
You don't mention any secondary physical mitigation layer like endurance charges or a basalt flask or any regular "less damage taken" sources like Kintsugi, Flesh and Stone, Wise Oak, Fortify etc. You also don't mention any avoidance layer either like block, dodge or evade. No physical->elemental/chaos as damage taken conversion. This is where the meat of the mitigation comes from.
It's the combination of multiple layers that makes a character tanky. Investing heavily into one of them (baseline physical reduction) is the easiest but also the most useless against big hits. Against massive hits your armor is likely only reducing the initial hit damage by ~15-20%, another ~20% from your golems, and the rest smacks you in the face.
Remember the layers of defenses. Avoidance, Conversion, Reduction, Mitigation, Effective Health. Combining those layers will give you much better survivability than just investing into one of them, which is why you're still getting oneshot regularly. A properly built character only gets oneshot by abilities that can be avoided.