I think there should be a spell where you recover your own mana using health. My life wizard goes for healing boost, power pip chance, and life damage, so the ring and athame don't give mana at all. Every so often, I need to recover in mana. And when I had to fight Freddy Nine Lives, I had to exit the dungeon and go back inside to get some potions. It was annoying because I ran out of gold and I had to go catch some wisps. Maybe a spell that recovers your mana by taking out 500 health and costs 0 mana. I lost so many times on my life wizard for that exact same reason over and over again.
First, giving up life for something is Death's medium. It'd be weird if Death got a restore mana spell and this point in time and all the other schools didn't get anything. By the time you get this spell, running out of mana won't be a problem.
A spell that gives you something for a cost in health sounds like Death school to me, not Life school.
If you keep running out of mana fighting Freddy Nine-Lives, you're doing it wrong. Probably you're using too many cards in your deck. You know exactly how strong he is and what spells he takes to beat him, you should have nothing else in the deck you use to fight him.