I think I found the problem. If you lose a 2nd time against the Willowisp you don't receive the increase to your mana. Therefore, you won't have enough mana for the next area.
I got stuck too. It appears to be straight forward. Hit fireball until they run out of mana then aoe. However I don't have enough mana for that strategy to work. I'm don't know what other moves I can make... : /