Currently I think the only way to get S+ rating in a level is to beat it without getting damaged (a lot harder than I thought) while maximizing your kudos by beating 2-3 enemies at the same time and using the ultra skill.
Discovered that there's extra dialog if you enabled Japanese voices.
But actually doing that is a lot harder than it seems. My timing should be split second accurate, imagine playing for 5 minutes and then you get hit and have to restart the level again, or playing for 3 minutes and pushing the jump button instead of attack button and get hurt by needles. Ouch!!
Not sure how long I will be though,
here are still other games in my backlog.