No big secret and not that high really, though I do like to have good HR.
She has an "... of the Glade" vestment which gives HR, a Hallowed Circlet of Regeneration, Demon's Blood with Energy Leech and Stun resist plus more HR on one of the rings and an Anhk of Isis on the necklace for a bit more. On the old forum I did run 3 toons using nothing but items giving HR as an experiment. A Diviner using mostly only green/yellow regen items with feasting/devouring/harrowing staves, an Haruspex with a mix of green and uniques and a Harbinger using uniques only. The Diviner and Haruspex have about 700+ HR, but the winner is the Harbinger - his unique items bring him 900+ HR. Looking at them in game just now though and it looks like things have changed a lot. The Diviner has over 1,180, the Haruspex just over 1,000 (bow) or 1,200 (spear) while the Harbinger trails badly at ony 500+HR. Bear in mind that these figures are with auras (ToC, Herbal Remedy) and also the uniques are socketed. And no, none of them are deathless.
Well done. Enjoy Act 5 and tell us what it feels like with Xmax.