Another way to deal more damage is to choose the suitable mentor for your Ultimate attack, like choosing Piccolo as your mentor and his last skill Special Beam Cannon as your Ultimate. This way you have a good chance to see a big Z on your screen while using the Ultimate attack, which will raise the damage output of your skill tremendously.
Keep in mind that you don’t need to equip the matching master Z-Soul to let this Z appear. The master Z-Soul is not responsible for “the Z”.