Okay. It's not great, but it's better than what most people around here call "dungeons". Your dungeon has no exploration mechanic from what I see. I'll let this pass seeing as you wanted to showcase just the boss for now. The aesthetics match some awful-looking puke, which is what I suppose you were aiming for.
Now, for the boss. It's not very dynamic. You have a mostly-immobile boss that's almost always unhittable surrounded by harass-heavy minions. The general idea of what you've given me about this boss is that it's punishing for low-range classes, especially the dagger-kind which have neither range nor defense to stay in the zone of harass. I'm not a huge fan of invisible enemies that chase you (the only way I could see them was by the shadows, and that's not exactly an excuse to make an enemy invisible).
You also want to avoid making "really easy parts" when you're making a difficult boss. Periods of shifting high power and low power are good for getting basic dynamics, but when you make the boss purposely miss the player, it's just insulting.
I would give you a 3/10. But to ease your conscious, 3/10 is better than what most people here can do. I'll give you the "MPGH-considered" re-rank of 6.5/10.