You could, technically, unlock this at any point, but there are several places where it is significantly easier than others. Once such point is in Chapter 9, after you've unlocked the gate allowing you to leave the large lake area. You'll swim through an underwater tunnel and eventually come to a place where your path is blocked by a very large stone. A large number of "pillbugs" will continually spawn for quite a while. Simply equip your favorite weapon - I recommend the Vigoorian Flails - and chain a series of level two Ultimate Techniques together. You should easily surpass one hundred hits without any problem as long as you use the strategy of leaping and pressing
as soon as you land to instantly charge your attack.