Is definitely think you should keep the super hard difficulty! I think it's kinda cool to have a difficulty that's really punishing as a challenge for those who want it. I'd maybe go the other way and add an easy for people who don't like a challenge.
Ohhh thats what the skull does! I was actually thinking 'ohh man if my multiplier didn't reset between rounds my score would be huuuge'... Shame I didn't put to good use 😂. Omg, keeping it a secret fits really well with the arcade theme, that's clever! Maybe if you did a tutorial you could mention it but only vaguely, like, "keep you eyes open for anything else that might make you fight easier" or "rumour has it there's a secret weapon designed to turn the tide of war!"