If you are going to test translation, you need to use English sentences that have clear meaning to English speakers. You still don't in this lesson. Get rid of Jack, the Giant and the beanstalk and come up with better examples. "Jack descended on the Giant" is only clear in its ambiguity without further context. I knew exactly the differences between être and avoir with descendre, until this terrible English sentence was in the quiz. 
avoir + descendu [quelque chose] ou [quelqu'un]= to go/come/climb down [something]