I'm playing bodd rush mode and had to figt Yex. From previous fights, I knew that Immolation was good at dealing damage to them and the creatures they spawn. So when I dropped into the arena the first thing I did was to cast Immolation. For some reason it didnt cause any damage. So I tried again. Then I tried to melee hit them. Still nothing. Yex was receiving no damage. The only damage they were receiving was the riposte attack from my sword.
Being a mage myself, I have run into this problem before, and I can say that the cause is likely the Hunter Eminent minor card, which causes only critical hits to deal any damage.
Since the Boss Rush chooses a card basically randomly, this is an issue we need to be prepared for, so it's a good idea to bring some harmless or beneficial minor cards to overwrite dangerous ones.
I understood what triggers it. If you damage the boss for the first time with a throwable weapon or a range spell, the only damage the boss now takes is critical.