William got his three spirit elements from the silver monster he killed and absorbed its soul before.
"I can't form clones, and I don't have any time to train over techniques that will generate clones. In theory I can't do it like that damn monster, but I have another way… I hope it works…"
William's idea was simple. Instead of making clones, he decided to use his spirit elements separately, leaving behind the one he wanted to activate in his spirit.
Of course, darkness was brutal and useful, but it was also used by his enemies. So it was foolish to use the same weapon his foes had, and it was better to use one that would negate the enemy's strength instead.
So, he got to select between light and lightning. The former was brutal and explosive, and the latter was deadly and got a purifying effect in it.
William knew if he used the light element quite well, then there would be no one to rival him even one grade higher. But he wasn't adept at all in using that element.