You wanna check out arcade for that (boss Nagoriyuki is all kinds of fucked), storymode is basically just a movie to wrap up the story of Sol.
And this game's best feature for newer players is letting people play with folks of their similar skill level below a certain point. The floor system is like their usual colored square system, only it comes with like a protective field which puts same colors in their own pocket dimension so unless a lower floor player chooses to go to a higher floor they will not be matched with someone stronger.
It does begin to crumble once you get to the secret 11th floor since in there you have people who range from folks who just managed to get there after a month by the skin of their teeth, to literal top level pro players, all in the same pool, but I figure anyone good enough to get to the heaven floor should be ready for anything by that point XD.