Sensei's big secrets - Also revealed!
Low blow: Get a score less than 20. You can't just not slice any fruit, because then the bonuses at the end will put you over. You have to slice 2 different power-up bananas (To avoid "No bananas sliced" and "Nothing but _______") and one bomb (To avoid "No bombs hit").
Triple stack: Have all 3 power-up bananas active at once. Usually easy enough.
Weapons cache: Unlocked 8 blades.
Scenic route: Unlocked 3 backgrounds.
For these last two, you'll have to buy some using starfruit no matter how good you are at getting achievements, because there just aren't enough.
Sensei's little secrets - revealed!
Single cut: Score 250 points without lifting your blade. If you drag your blade off the edge of the screen and then release, you won't even need to hold the mouse button down.
So close: Score one less than your current high score. My suggestion is to do it at the beginning of the week when you have a fresh start, and get something as low as possible to minimize the margin of error for the end-of-round bonuses. Speaking of which, don't forget to take those into account - you're nearly-matching the base score, not the total score, because you'll be getting the same bonus points this round, too.
Happy ending: Get a critical on the last fruit of a game. You can't control when you get criticals, but if you're impatient, you can change the last fruit. Just stop at a critical close to the end and make it the last.