An EX Special Move is a more powerful version of a characters Special Move, using two attack buttons instead of one.
Do use an EX move, you must first fill up at least one section of the Super Meter. Then input the command for one of your character's special moves, and instead of just hitting a single attack button, it's easiest to just use the LB or LT for "ALL" as the final button press. The move should be more powerful than a regular one (I.E. C. Viper's Flame Kick will add a roll for extra hits in then air before the kick). Simply do this 100 times to get the achievement. It should come just in playing the game as long as you remember to use them.