Unless you're a rhythm game expert, I'd say the most challenging achievement is getting 15 Ninja Arts or higher. Guides will point you to the best opponents for getting this but here is a more comprehensive table of the highest Ninja Arts you can get per opponent per difficulty, and how much leeway you have.

Now you will know what Ninja Arts you can hope to aim for in each case for that achievement, and challenge yourself further after the achievements are complete! Now that you will have improved in the game.

The format is Max Ninja Arts - Notes Leeway. For example:

Yomi on Normal: 15-14
The highest Ninja Arts you can get is 15
Providing all other notes are Perfect you can get 14* Greats or start Ninja Arts 14 notes late (or a mix of both) and still get the highest Ninja Arts possible.
*If your Greats are in a row you can afford a few more possibly upto double.
Of course a Bad note (missing the note entirely) resets your Ninja Arts to 0.


I have also included this info in a spreadsheet[docs.google.com] which I used to keep track of my personal bests if you'd like to save it, clear my records and record your own best.