As much as I feel Beowulf is the most thematically fit for Grand Berserker, I have to admit this opening showing the Twelve Labors (I still prefer the HF version) is probably the best possible to showcase a Grand Berserker. You are legit seeing the tribulations he had to do in life in sequence. Other heroes may have more absurd deeds but I can't remember any hero with as much fame and as many as Hercules. Whenever I imagine other people it goes from 2~5 at most. And people that had to do many trials don't feel nearly as significant or famous.