Walker was observing Midnight's attempt to absorb the light elemental mana similarly to how Onyx did and even attempted to breathe some in to absorb it that way. Meanwhile, Alice was showing off how she could attack with light that she personally condensed. Su had realized that the attacks were field by the mana in the air and not by Alices' mana completely. This meant that Alice was using her natural stamina to attack and no skill was behind it.
The thwacking sound repeated over and over as Su had dropped her shield to block using her own body. Alice lacked strength and magical attack power. This was simply due to her weaker level compared to Su's magical and physical defenses. But this worked out. Su was able to avoid being damaged while training herself against random attacks and attempting to resist the light elemental mana that would burst from Alices' attacks.
