Breakthrough! But…
Xavier had depleted his mental energy. Before Xavier and Eulene could even venture into the haunted gallery, Xavier had used a lion's share of his mental energy on preparing that devastating move—'Death of a Star.'
Then, in the room full of paintings, the trick he pulled off against Walpurgis depleted the scarce mental energy even further. He barely used any energy in the Mechanical room, but the library was like a black hole to his mental energy.
His mind had become utterly exhausted after remembering the exact page, line, and word from seventeen thousand of the books he had memorised. The internal competition against Eulene only added to the depletion.
Xavier had entered the core of the haunted house, the 'boss room', with the bare minimum of his mental energy. He had no choice but to use an external weapon to conserve his energy.