Number 12, marked Allegro assai, should fly off the fingers and at least give the impression of ease. After hours of practice, I did find that this piece began to feel enjoyable at a fast clip. As with many of Campagnoli's caprices, you cannot find even one dynamic marking: this gives the performer creative license with dynamics and shaping.
With the long slurs, the focus in this piece is definitely on the left hand. The left hand should feel as pliable as possible, as often shifts happen fluidly: an extension of a finger, followed by hand adjustment. Measures 4-6 are a perfect example: the first finger reaches back and the hand follows suit. I found the most difficult passage to be mm. 21-26. I devoted a lot of time to backwards practice here!