Level 100 Hybrid MoM Crit Arc Elementalist^3.19

Notes

Main build concept: using Olroth's + Summon Skeletons + Heartbound Loop to generate a ton of "taken Damage" for Nine Lives and other sources of Recoup: we get insane Life, Energy Shield, and Mana regeneration from this.
Since we stacked a bunch of Mana Recovery Rate as well, we get something like:
(350 * 2 Heartbound Loops) * 4 (number of skeletons dying each Summon Skeleton cast) * 13 (number of casts over 4 seconds) / 4 (to make it once per second) * 0.62 (physical damage reduction) = 5642 Damage taken per second.
But all of that is taken by our Ward, so we don't suffer at all from this.
Since Recoup is over 3 seconds thanks to Nine Lives, we get 5642 * 1.7 (Mana Recovery Rate) / 3 = 3197 Mana regenerated per second!
Naturally, this number may vary depending on other conditions, but if we fine tune it with our mana spending, we can easily ramp over 10k Mana spent Recently and get some nice sustained Indigon value without investing a ton in mana regen alone!
Instead, we invest in Wardloop, which gives us big defensive benefits (though since we use Indigon, we do lose around a third of our Ward).\

However, since Wardloop was just nerfed in 3.20 (namely, the 'reduced flask effect' Survival gem was removed, meaning Wardloop's permaward value is a few hundred less than before), this defensive layer drops a bit in value.

Damage can be used with whatever can be stuck onto Mjolner (since Mjolner doesn't cost any mana with its triggered spell), or one can choose some other cheap spell and finagle the cost that way (since Summon Skeleton's mana cost
isn't hogging all of it for itself). It would need to be very cheap though, no more than maybe 140 combined mana spent over 4 seconds (including support gem multipliers), since Cyclone is around 2 mana for 15 casts (30 mana total)
and Summon Skeletons is 17 mana over 13 casts (221 Mana). The goal is to have these add up as close to but less than 200 / Indigon increased cost modifier (usually try to shoot for 0.5) = 400 (usually) as reasonably possible.