When facing someone who is hard to kill, it is sure convenient to remove his easy means of getting healing (Harm) first. Roy is correct to be skeptical that Julia's idea can be implemented, while he knows it is a good idea.
Once Xykon is gone, Redcloak will be running out of options, with destroy the Gate himself or negotiate being the only obvious ones. So, it becomes a simpler tactical problem of convincing Redcloak to not destroy the Gate quite yet.