In level 14, fight 100 opponents, including all the troop types from the game to unlock the Ultimate Saber. The Ultimate Saber enables one-hit kills on all enemy characters excluding Bosses. You can turn it on or off in menu that appears when the game is paused.

Note: You only need to get it once with one of the characters for all characters in that mode (one or two player) to be able to use it.