This is a fairly simple achiemvent that will come over time.
Simply unlock 10 "hidden" items from Sensai's Swag.
There are many ways to unlock the items, and more than 10 available.
To see a checklist of what you need to do to unlock items, go to :
Main Menu > DoJo > Sensai's Swag.
From here, you can see the unlockable item - for instance, Fire Blade. Select it, and if you havent yet met the requirements it will tell you what to do - i.e - "Hit 20 bananas during Freeze".
Simply knock 10 items off the checklist, and you'll get it.
I unlocked this achievement through normal playing, without checking the list once. So, I wouldn't worry about it too much.