You can just unlock unlimited's by beating Score Attack with the normal character (Lambda in Nu's case).
Granted, I can understand if you do not have confidence in being able to do so xD
You can set up a second controller and press start on it whenever you're about to lose in score attack to cause HERE COMES A NEW CHALLENGER. Beat the player match, then you'll return to the same place in score attack, but playing from the beginning again. You'll eventually get there with enough work.
Also, if you don't feel like working at all, I looked on Xbox Marketplace: Unlocking ALL unlimited characters costs 400 microsoft points, so... 4 bucks.