Part of war propaganda is to assign a nickname to the enemy so as to dehumanize them. The USA has done this for decades in the wars in Southeast Asia, Vietnam, the Middle East, Central America, etc.
Did the Japanese soldiers refer to Western soldiers as "鬼畜米兵"?
Did the civilian population also refer to Western soldiers as "鬼畜米兵"?
Was there a slang term used by Japanese soldiers to refer to soldiers from Asian countries in WW2?
鬼畜米英 is somewhat famous even today as a slogan, and this is what most of Japanese people learn in history classes at school. But this phrase is obviously too long for everyday use, and there seems to be little evidence that this was widely used in speech during the war.
Other shorter derogatory nicknames I know include アメ公【こう】 and 米鬼【べいき】. In particular, I've heard アメ公 used often in war-films (which were created long after the war), but I admit I have no idea how often these were actually used by Japanese people in those days.
Wikipedia has the list of derogatory nicknames referring to people in various countries. I'm not familiar with some of these, though.
