Erdrick is a pretty cool Dragon Quest main character and definitely one of the best ones in the series. He has an array of elemental attacks as well as physical moves that he can dish out for massive damage. Ryoma would be wise not to let Erdrick get anywhere near him. Ryoma has always been pretty good at adjusting his play to counter all potential attackers so I don’t think that will be a problem for him. Ryoma will keep winning as per usual. Ryoma wins.
Update 8 7 2020: Ryoma won’t be able to stop Erdrick’s magic. Erdrick wins.