"Soul of Light? How wondrous it is. If I can mobilize my Illusory World, what would this mere Soul of Light count for?"
Merlin felt somewhat powerless. Clearly, he knew that Soul of Light was illusory but he could not locate its flaws, so he had no way to dispel it. It was like Merlin's Illusory World. Once it enveloped his opponents, even if they knew very well that this was illusory, they were powerless. This was the scary part of illusions.
Of course, compared to the Illusory World, Doomsday's Soul of Light was lacking by far. Nonetheless, with Merlin's current state where he could not mobilize his Mind Power let alone the Illusory World, wanting to dispel the Soul of Light was challenging.
Thus, Merlin might as well stand there quietly, shutting his eyes calmly. He was patiently searching for any flaws in the Soul of Light or the place where Doomsday's real body was.