Use "special weapon upgrade points" to level up Michelangelo's Kusarigama three times. You earn one special weapon upgrade point when you complete level 1, another point for level 2, etc.
One fast way to gain 3 special weapon upgrade points is to apply ability points, when you level up, to Donatello's "The Genius" ability. You can automatically gain 3 special weapon upgrade points. Put those towards the Kusarigama, and the achievement is yours.