My strategy was to pick only defense to make soul vials (which seem to grow like mushrooms in this dungeon ^^) more valuable (you heal "more hits").
And then killing every 'g' and 'G', avoiding 'D', and gathering all the soul vials of the level.
Indeed this formula explains what I was not understanding.
So I think choosing defense is really good at the begining. But always take defense is less and less impactfull. Maybe switch to attack at one point, to avoid taking some hits by killing faster ennemies.
For health, I don't know. Since we level up relatively often in this game (even in late game), it might be good too, since you heal full health at level-up. It depends of the gained amount. I would say at least 5/level-up pts maybe 10. Because soul vials lose value the more max health we have, and the less defense we have.
To test the boss, you begin the game in the boss room with ~ 20-25 skill points and the best armor and the best weapon. I also had 4 vials I think. (to save you the need to cross the whole dungeon).