Liang needed as much power as possible to completely fill the sphere - so he chose the easiest option. In order to do so, he needed to release the power that had been lurking within him the entire time.
Only during the fight with Nitra, did he reveal his Form for a moment, and now he hardly needed more time. Liang's full strength was truly crushing, and if he kept his Form active for long, it would only lead to unnecessary destruction.
During the battle, it could easily help Liang win, but right now, there was no need for it. There were no enemies around, only spectators and even friends, so he just wanted to deal with this problem as soon as possible.
Nitra, Alucard, and Gael had created protective barriers dense enough to withstand the Dragon Elder's breath. However, no one was going to attack these barriers, it was all necessary to withstand the aura and energy flows coming from Liang in chaotic ways.
