Firework seems to be a quite useful and essential utility spell for a warrior, but I can’t figure out which runes are useful for it.
I tried to upgrade its duration, which is a bit short by default, but it seems that while the firework itself does exist longer, the enemies still lose interest in it as quickly as before. As for the area, the extra range rune doesn’t really seem to expand the aura range at all (I might be wrong about this one, since it’s hard to tell distance in this game). As for the range, if you can’t see that far yourself, what’s the meaning of increasing the range of spells?
As such, I think that I’d like to improve this spell a bit, but none of these runes seem to actually help. Did I miss anything?

Just finished the game. Mixed feelings. It turns out that the practical “fireworks upgrades” are actually outside of the firework crafting:
1. Improve your Action parameter by upgrading your Action or Dexterity skills (the former affects more), or build yourself some Haste-imbued equipments, or, both. This makes your spell casting faster, so you can move between casting fireworks more easily. Also, backstabbing will be a little bit faster I suppose;
2. Use Invisibility spell or item, so that you can lure the enemy closer to you, which makes your running after them easier.

This game just makes quite a lot of weird design decisions it seems...but well, at least it’s unique in some way and I don’t see so many other games that’s like it.