• All stars are mandatory. No matter if you miss last level by 0.01, that's instantly B rank
• Number of deaths and repeats are limited:
• With 0 deaths you can restart 7 times
• With 1 death - 5 times (meaning 1 death and 4 restarts without dying)
• With 2 deaths - 3 times (just one restart without dying)
• 3 deaths and more - A rank and lower

Feels like it can be counted as (Deaths*2 + Retries), where every death also counts as restart. If result is 7 or lower, this is S rank, anything higher is A rank or lower. All stats above were tested on bonus marathon, but should be the same for other worlds

Additional note:

• If you complete the level, ninja dies and you select "Next level", that doesn't count as death nor repeat
• If you complete the level, ninja dies and you decide to repeat level (for example, if you've got 2 stars), that's counted as death + repeat. Be careful!