Solo variant of the more group-friendly version here. The full three umbral forma is only to support build variety, you get away with two umbral forma and polarizing the exilus slot, but you'll lose a lot of variation potential as described on the page for the group build, and one if you polarize completely for this build and sacrifice even the normal group variant.

If you stick to Rejuvenation and Coaction Drift for the aura/exilus and use an archon shard to give [Nidus] another +5 health [Regen], he'll have 22.5 per second without the extra healing from Ravenous (60.5 with it,) as well as 85% damage mitigation without Arcane Guardian proc'ing and Parasitic Link hooked to an enemy. Twenty max-level heavy gunners can just slightly outpace the health [Regen] through all the mitigation with Ravenous and Parasitic link up, and outside of the simulacrum, there's always going to be a mix of enemies that do way less damage. Also since you'll be killing them (and crowd-controlling them with Larva as you gain stacks in the first place) because you're not sitting there just testing the incoming damage, in real gameplay you can easily swap out one of the Carapace mods for something else, maybe an augment of your choice.