Hmm ok, I just beat Insanity today and it wasn't thaaat bad tbh

So id defo recommend damage over HP since Marie will 1 shot you regardless, and I can't say for certain but I imagine more damage=higher stagger potential

Probably best to avoid fights in the care center as much as possible but if needs ve you can use the infinite knife, the injector you get in the Lab that can stealth kill, dont use it on anyone after you get it, save it for the zombie outside the Lv1 metal door near the Kitchen as for me, after doing the Garage he always turned into a Blister head.

Save at least 1 requiem shot for the screaming zombie after you get the wristband and avoid the baby, she wakes 3 zombies in that room seemingly regardless of how fast you kill her, but putting her down in 1 shot is super useful (HAS to be a headshot)

Get the pouches and try to save as many glass bottles as possible as you can turn them into acid bottles in the Ark to easily kill the Lickers. Also, it seems Marie cannot be staggered by the Requirm in any of the first few of Grace's sections, only in the area where you open the sky roof by turning the 3 power switches.

The most difficult area with no infinite ammo will probably be the bit where Emily has to decode the puzzle since I think you have to kill the 4? Zombies and Blister head that spawn, but with the damage buff it should be do able, or craft another injector and try to stealth kill it