Sit Down, Son!
William was feeling frustrated. His injuries were healing at a slow rate while the Giant Golem in front of him was wreaking havoc like a little child throwing a temper tantrum.
In short, the situation was very tricky, and his mind was working at full capacity in order to think of a way to deal with his current dilemma.
It was not as if he couldn't beat the crap out of the Giant Black Golem, but the problem was that the entirety of K-City was filled with people.
Fighting against the Giant Golem would require him to devastate the surrounding area, which would lead to thousands of people dying. This was something that the Half-Elf couldn't do, so his hands were tied.
The Giant Golem, on the other hand, didn't have this problem. It was a creature that was born to destroy, and take the lives of others.