Alysis hadn't infused her lightning just to prevent it from being dispelled; she had also lightly bonded the lightning's existence to her arms. Using the holes she'd blasted in the walls to give her a line of sight, Alysis twist her arms to alter the lightning's trajectory, bringing it back into alignment with William's position. If it hit him, Alysis's Existence Origin would allow it to persist, so it would continue to disrupt William's internal processes beyond the initial shock.
Meanwhile, Saber noted Caster's attempt to sidestep. He allowed her to move to the side, and then when she started to raise her hand, threw himself forward, dismissing the turn that would bring him into the path of William's swing in favor of a straight forward tackle, aiming to ram his shoulder into her throat.