What you've described is the very first fight of the intro. One that, mind you, can be soloed by Vectron himself without even having Alexander in the party. (I've done that several times when testing, though granted it's not 100% win rate with only one character.)
The whole Demon Queen boss fight is designed in such a way that making it even easier than it already is would be actively preventing people from the ,,actual content" you mentioned. As for the fights you have any trouble with, Wind Lashes spam + Strong Attack (true, RNG based, but it's the identical one the entire Adventure Call was built on) cleans up everything with barely any damage done to the party (dragon is an exception, but you get 100% healed after it).
And idk what you mean by ,,no healing", you literally have a character with a heal spell. Not only that, she even has an amulet that regenerates HP if you don't want to use any of the potions or wine that you loot in the castle.
Either way, that's literally just the intro. You don't use RNG based attacks afterwards, and you'll get plenty of opportunities to find healing items.