hmm...I think that depends on just how strong Dyna's Strong form is in comparison to Beliel. You're definitely right on Miracle. Thing is, the movie wasn't about Dyna, it was about Zero. That's something we need to keep in mind.
dyna took tiga's forms and made them better (I have nothing against tiga) does sky type have pcychic abilities? miracle mode is fast and psychic dyna's flash type "+" finisher doesn't need charging either, he only charges it to give it even more power
In the fight against Beliel, I think Strong form would have been useless, Miracle form, I think Dyna would have been able to dodge more of Beliel's attacks, but wouldn't be able to do as much damage as Flash type.