it was a rock with blue crystals
small rock from level 3? or the one with lots of crystals in the last level?
it's very strange, the small rock cannot be hurt with the level 1 sword, only with bomb or level 2 sword, but all Melee monsters should at least do 1 damage to the Hero.
As the the big crystals one, it has a base attack of 7.
i may have messed up something when debugging the use of potion during fight the last day.
Damn, i found it, i inverted a 0 an a 1 on the last minute code i did ^^. Time to learn how to compile a patch with Godot.
The game check if the hero has potion, but try to use it from the rock, so hero never runs out of potion and can't die...
btw, you can kill it with 1 bomb, but this bug may appear elsewhere.
i think i didn't notice because hero can use potion between fight too, so even if he goes to -3, he can resurrect.