The lack of renewable healing is an interesting concept, but really doesn't work in the long term because the less skilled players who need the healing will run out faster. It also encourages just dying to respawn if you take too much damage. And even if you manage to get around it somehow with incredible level design, the level editor version won't be like that.
The first boss feels like its attacks aren't telegraphed or reasonable to avoid, but its health is low enough to just tank it.