On learning the Crestfallen Cool spell, going to obtain Life Capsules is one option...
Using the only Life Syringe in-game (obtained by clearing Ririn's quest) increases max HP by 10 (starting max HP is 4; maximum HP by end-game, likely 160).
Crestfallen Cool is powerful, at Level 3.
That's good to know. Considering that you can get 160 HP in total, I take it that the game's difficulty will still be challenging even if you get them all?