ALL the DBZ movies are around 50 minutes, most are less though, like 48 minutes, and 3 of those 48 minutes are used in the introduction. So you could say that a DBZ "movie" is about the length of 2 normal episodes on TV. About your question, the developers didn't want to spend too much money, so all the DBZ movies are low-budget, which means that they don't want to spend precious money making a guy transform 10 times just to kick Goku's ass. You can see Goku get his ass kicked in Movie 8 by Broly(or Broli if you into Japanese names), if that is what your looking for. But in the end, Goku finishes Broli with a single punch. His Kamehameha does nothing to Broli, but this single punch to the stomach defeats the "Legendary Super Sayion".
Live with it man, all the movies have cheap and rushed endings, it's something you gotta get used too.