For the most part there's alot of luck with this one. Evidently Dynadmon can be paniced but it only works some of the time.
Honestly I won with UlforceV, Craniamon and Mastemon, with character reversal on Craniamon. Start by reversing Dynasmon, Chaos with Mastemon.
Then spam Breath of the Gods with Craniamon. If you're lucky Dynasmon will kill himself since Crusadermon heals infrequently. Worked for me firsy try. Turning off battle animations are mandatory. This fight was a really though nut to crack onhard.
The balance is shit BUT most of the Royal Knight bosses have some glaring weakness. UlforceV which you had trouble with is trivialized by poison, remember all the antitidote moves he spams? Some later bosses on HM demand particular attacks and strategies, often like SMT megabosses.