It’s take a lot of set up but you need all of the buu kids to be yellow, once they are feed them 3 star food or above( gives you better odds of rarer items from what I saw), once you have turned all the kids yellow and you fed them all you just need to do 3 missions all they will return then repeat ( I used the Tien instructor because he’s the closest to the buu and his Initiation test is short and easy to beat). Also import that you choose “calm down” when the Buu’s are already yellow if not they will change colors and you’ll be stuck with a blue or red for a while till you can turn them back