(I'm in normal/medium difficulty mode) As Indinera and Eridani pointed out, it's a good thing to have lots of supplies! We entered the crypt with Random at level 23, but by the time we got to the demon, Random was at 25. I bought more than 50 each of Medical herb, Health Balm, and Magicka Tea at the first Lamia camp. I certainly didn't use all of the supplies, but I used a lot. In my case, I think I succeeded because I had enough supplies this time around and my level wasn't as low as when when I entered the crypt the first time. Also, whenever one of my team HP got close to or below 100, I made sure to bump them up - just in case.
Well, this was just my strategy, I'm sure there's lots of other ways to get through it.
Hope this helps and good luck! 
