Calling this game out for being derivative is besides the point really, it makes no secret of its main inspiration. Indeed, it is in all likelihood to be the game's biggest draw, as it certainly was for me.
Unfortunately I did not enjoy anything about the game besides some visual design elements.
There's clearly a team of passionate and talented people behind this, I just hope they find some fresh inspiration for their next title. I'm completely aware of the contradiction I'm making in criticising a game for being the thing that drew me to it, but it I really did not enjoy the gameplay or structure. On paper, the 'harden' mechanic is just interesting enough to give the game some pull, but the execution is lumpen and offers scarcely any advantage.
After finding all the shells and slaying a few bosses I put it off for the last time. While my experience was unsatisfactory, I still look forward to seeing what the studio come up with next.