Fly away from the boss until he's about 1.5 - 2 screens away from Goku. You can keep shooting the boss with Ki Blasts from there and the boss won't move. You can find a place to shoot by firing Ki Blasts in the boss's direction and moving side to side until it makes the sound that it makes when you hit an enemy with a ki blast. Once you find the place where you hit the boss, stay there and keep firing Ki Blasts. It works because bosses don't move when you are a certain distance away from them. For some bosses, after you fire ki blasts for a long time, you might need to go up to the boss and punch him/her because there are some bosses where you need to be on the same screen as the boss.