Just do the square, square, square and hold triangle until the bar is full. ( Note: Do it in SSJ. In normal form if you try to get ssj2 aqnd the ssj bar is filled you've wasted valuable time trying to beat cell.
What I did was I just kept attacking cell and just kept hitting the crap out of him until I got ss2 and knocked him out so try the same thing but it may not work for every one so just try it once and NEVER DO IT AGAIN!!!
This game is not very good. Ya sure if you play it after a long time you might find it a little interesting but if you plan to get this game then please rent it from a friend or somthing before you buy it. I hope you use my advice.