I noticed that some players were starting battles with his Ultra ready on the 1st round, so i tried to figured out how they can do it, and i got results.
If you start any Quest Stage and use SP potions or fight with enemies to fill up your gauge, and after the gauge is full, if you open your chat window and open someone profile and challenge to a practice fight, you'll start the fight with your Ultra ready to use and also affects the Cheer Bonus. But, the glitch still work even if you leave the fight, and it affect others Practice Fights, Arena, Heroes Hall and Relic Hunt, and last until you close the game.