Play the military droid and lose. Then set the options to the following:
Difficulty - Hard
Timer - Off
Bouts - 7
Cinematics - On
Shadows - On
Screen Shake - On
Now select two players, and on the handicap screen push left so that the red bar moves about quarter into the player one side and back again about six times until the screen flashes. After this has happened, cycle through the opponents and after the sentry droid, there she'll be. Player two can now play as her, with special moves being; down, towards, up, for a mutation, or; down, back, and up, quickly, to melt and reform.