ANBU really are strong! In the series they are considered very skilled.
The only reason that they die so fast is that they got ambushed by kabuto. Have you ever thought about that Kabuto is the only one that we see in the anime/manga killing ANBUs, and we all know that Kabuto is strong. He's as strong as kakashi as oro says. and kakashi is strong, this guy became a chounin at the age of 6 ffs.

About the jounin exam. I think it's a promotion you get by the council when you show exceptional skill and stuff like that.