The easiest technic to do this achievement (and the 60 Hits achievement) is to use "Franck West". Go to the Arcade/Online/Lobby mode, select "Franck West", save up three bars of specials and perform a lv3 hyper combo. (Combo name in the list "Say Cheese" Hyper Combo LV3 ) 100 HITS COMBO! Very easy to do. The achievement pops at the end of the fight.