Lowering Immunity to Fear?

[ExLibrisMortis]

I'd argue that even a lowered immunity is still an immunity. However, you don't need to lower it; you can simply deactivate your Aura of Courage (Su). Its activation action is not listed, so it defaults to a standard action. I would let you deactivate it as a standard action, as well--essentially, the whole aura is a toggle ability. If that doesn't work: Aura of Courage is a supernatural ability, so you can suppress it with antimagic, and it is not active while you are unconscious or dead (although the latter clause is probably hard to leverage).