I think my only complaints would be the second and third boss as make the fight drag on for way longer than it should be but that just a minor complaint for me. Also some spoilers for stage 3 below but might help you out since you are kind of expect to read Japanese to progress.
Stage 3 Spoilers In stage 3 the saves will turn into black skulls, and you see a a portal that is blocked off way above you. There should also be a purple border covering a button and for you to progress you have to stand on the button, press Up and you should be taken up there in a new character. At that point, you have to get up to the portal in the certain amount of blocks that you have, listed in the top left. Going to 0 will kill you and you also jump way shorter than you would normally. Its basically a puzzle for stage 3 but nothing to bad.
Overall I would recommend this game to beginners looking for a challenging adventure game that isn't something way to out of their league.