Yes I do think the rank might be a little unforgiving. I took my own average score throughout testing and gave myself a "C", but now you know if you get better than a C you're better than me ;)
Yeah some of them prolly have a bit too high in HP near the end, and the enemies near the bottom are annoying too. If I build this game out further I want to consider having more live movement in enemies that sink to the bottom and come back up, but the stationary ones will stay near the top.