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.