"I don't know!" William yelled, crawling out slightly further onto his branch and wrapping his arms and legs around it securely enough that he could handle his wand. So it was Nate, then, which was both good news and bad. Nate knew how to handle himself around Dark things - if that was indeed what had infected Gerard - but William had been hoping for, say, an entire crack team of Aurors just Floo'd in from Edinburgh.
He didn't want to hurt Gerard badly; it was still Gerard in there, he was sure of it; something had gone terribly wrong but that didn't mean the situation was irredeemable. He sent two Stunning spells down after Gerard in quick order, and when that didn't appear to work, he tried a simple trip-jinx and an Immobulus, with similar results.
Just as William had developed wandless magic for a brief time, it appeared Gerard was now shielded somehow. Well. William might not be able to affect Gerard, but he could still affect Gerard's environment.
There weren't enough vines around to make practical use of, but there was a young sapling still supple enough to wind around Gerard's leg, and a tree branch willing to be bent to his will, thumping down heavily onto the ground and narrowly missing Gerard's head.
Which gave him an idea.
They couldn't stay here; Gerard would become angered enough soon to start climbing after him, and Nate was on the ground with no cover. Running further into the forest was a dangerous idea, and Gerard would most likely catch one of them before they made it far, at which point... William chose not to take that particular avenue of thought. Leading Gerard to the castle would be disastrous, even though the promise of reinforcements was tempting. There was really only one place he could think of for them to run.
"Nate!" he called down, bombarding Gerard with a small handful of rocks and praying it would give Nate time to find cover. "The willow!"