Ultimate Saber: 

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. 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.