When I was a teenager I read a book about a girl who lived in a world where there was magic, but magic was not all powerful. People could do limited tricks on their own, but for great feats a circle had to be formed.
In the books it was made abundantly clear that not a single magician could best a demon, only a big circle could hope to accomplish that.
Demons needed to be fed bodily fluids to stay bound/connected/controlled which would weaken the magician having hold over the demon making the possibility of the demon overcoming the controlling mage bigger.
She went to a magic academy and at some point a demon was set upon. She got in a fight with the demon and discovered she had huge reserves of powers making it possible for her to best the beast.
At some point she confronted a mage controlling demons, which held the shapes of lions(I think), where the wizard had sexual relations with as to provide the required bodily fluids.
Somewhere in the story she had made a pact with a demon I think. The demon sweet talked to her tried to convince her he wasn't that bad.
Another thing I remembered
At some point in a battle she tried to repeat the trick she did with the first demon battle she had, but she couldn't access her reserves, she was too weak or something or couldn't access those reserves.
I read this book in the 2000's ish I think, but i'm not really sure.