Go to the majin buu saga. There is a scene called training with goten. You must be able to beat gohan with gohan. (He is pretty hard to beat with goten....... I had to go super saiyan with him).

GOOD LUCK!

woopsies on the answer i wrote beat gohan with gohan. i meant to write beat gohan with goten.

well in the buu saga when goten is fighting gohan add alot of health capsules to goten then you can beat gohan to get destined rivals

if u lose u get the money but not a new stage it is a win win game but if u want a new stage u hav to beat gohan with goten

defeat gohan as goten in the majin buu saga to unlock destind rivals scenerio hes hard to beat but not 4 me just have a good set of z items (i dint have to go super saiyan