Michaux had already led his casters to cast buffs on Guan Dongyang's spell. They couldn't leave everything to the Inferno Hell, since it would be inadequate to fend off the Zergs' attacks. These Zergs were powerful, and not even the forbidden power was capable of wiping them out, much less Guan Dongyang's spells.
After the Mastery casters had finished a group spell, all of them would be too exhausted to carry out any other tasks. Therefore, there would be no point in reserving their power right now.
The fire danced and sputtered on the battlefield, lighting up the moonless night sky. The Zergs seemed to have sensed the danger that was about to come, so they picked up speed as they charged at the human soldiers. The Zergs were much better than the humans in controlling their urge while on the battlefield. However, they were unable to follow their instructions and position while the situation threatened their lives.