There are 2 possibilities for your situation.
1- you used all your potions before reaching the boss and didn't notice the cache of potions before the boss
2- you have trials on. and one of them make the potions only health 1 and there is another one that removes supply caches.
check this 2 things,
TIPS : the demon is pretty easy, even without health potion. use your mark in the beginning of the fight. this will make him vulnerable from the beginning and you will gain time. Thenwhen he uses his invulnibity again. make your companion focus 1 monster and make Casandra use challenge on the other one. that will make you free to use the mark again. repeat it for the second wave. close the rift. you win