I found redemption from phoenix very useful for surviving/mana
x4 facet monarch
20% fire damage
Phoenix
-8% enemy fire resist
50 life
Fire absorb (helpful generally with your sunder and more helpful if your merc uses cure, I use ff for merc but have thought a lot about moving to cure)
Redemption aura
if 20% fire damage gets you more damage than -8% fire res I'd still take phoenix for qol
to run numbers to put it into perspective, say both setups bring non-immunes to -100% (70% or less resist in most cases so yeah), and you use the same 6 facet sword, enigma, d-fire raven, +2/fcr craft ammy, soj, opalvein, magefist, arach, 7x skiller, +15% sunder, torch anni cta, infinity merc
20/20 shield setup is 7099.2 / -80 and deals {14198} to non-immunes and {7241} to immunes
phoenix setup is 6310.4 / -88 and deals {12621} to non-immunes and {6941} immunes
Overall that's 12.5% more damage vs non-immunes and +4.3% vs immunes for the 20/20 shield vs phoenix
Its a significant but not crippling damage loss for QoL there, might on average be ~10% more overall damage on average since monsters are a mix but mostly not immune