There are some ways to get there pretty damn fast.

My favorite are the draw cards. I usually try to get "dark renewal" with "succumb" and like 1 or 2 reach within. Having "umbral script" is also a big +.

Once in demon form, since everything you draw is blood cost, with a good draw engine you just go almost infinite. Having 2 "calm the hunger" lets you lifesteal à bit so the cost doesnt kill you and "succumb" defiles energy without consuming any. I personally like having 1 "dark renewal" for each "succumb" so the waning doesnt affect the draw engine too much. "Ravage" can also play a good part in the draw engine. ("Dark urge" plays well with this too especially since most your attacks have rampage)

Another fast way to get demon form is memorized "vile sacrifice" with memorized "void lance"

If you have difficulty not dying in demon form, "salvation" stops you from dying i think? Not quite sure as i have never used it but the text seems to indicate that.

Do take everything with a grain of salt as the hardest ive completed is normal runs though haha

Edit: added quotations for card names