I think Ms. Gail got some real bad advice in school. To truly master a language, one has to THINK in that language. Translation is the last concern. I think in English when I write in English. My French teacher said the same and he is perfectly right. Subtle things in a language are not translatable. You have to FEEL them. As Robert Frost once said, poetry is what gets lost in translation. Frost was a master of language(s) and even he could not truly translation poetry.
People should reflect on what they are told and decide for themselves whether the advice is good or bad, right or wrong. Now Ms. Gail has long graduated from college. Maybe it is time to reflect.
There is time for everything under heaven, including slang. What Bugs Bunny said is not even a slang. It seems that Ms. Gail still hasn't bothered to find out what Bugs Bunny really meant to say: "What an imbecile. What an ultra-moron." I don't see any slang here. In fact, it is just the opposite--the words are too big for him.
Bugs Bunny and his friends all talk like a two year old whose speech facility has not fully matured, so they will sing songs like, "I am working on the wail-woe..." (Hint: nobody is wailing because of woe). Words like "imbecile" and "ultra" are simply too big for them and difficult for them to pronounce. So Bugs Bunny got the pronunciation wrong. It is like hearing a two year old struggling to imitate an adult who is quoting Confucius. Yet amazingly he got its meaning right. This is humor. Of course, people don't analyze it this way when they hear it. They just laugh.