To do this easily, you also must have another achievement available that you can easily unlock during the normal Story mode game (for example, "About five floors high" or "Marksman"). Enter Training mode during intermission of Story mode. Practice until you get a 999 (or other required number) combo, then exit Training mode. Begin playing a normal level in Story mode and earn any other available achievement. After earning that achievement, the achievement(s) that required the combos will also be earned.