What do you call a monster with extreme magic control ? What are the difference between one and a "normal" monster ? Do monster with very little magic control exist ? Are you saying that River and/or Gaster have extreme magic control or that they are medicals professionnals ?
Both River and Gaster are medical professionals, but River is also one of the rarer monsters with extreme magic control.
While magic is essentially what all monsters are, some learn to better wield and control their magic, as well as learning many other types of magic besides what is natural to their monster type.
River was the Royal Scientist, but that title is more modern. Their role use to be called “Royal Warlock” and it involved a lot more than just being a scientist. It heavily involved being a master at magic, knowing almost all form of magic, from alchemy to seals to healing and etc. Someone who wanted to be the Royal Warlock not only had to be academically smart and highly intelligent, but they also had to train rigorously in magic control. The Royal Warlock was always one of the best when it came to magic use, along side their Queen and Captain of the Royal Guard. There’s a reason the Royal Council was one to be admired and why their enemies would try to target them to take them out first in the war.
River was not only brilliant, but very powerful. Not like Toriel is since shes a Boss monster with high LV, but close. Their magic control was one of the best. They would never accidentally hurt anyone with their magic or make mistakes. (Like how in the actual Undertale game Toriel can accidentally kill you with her fire. That would never happen with River, or heck with our Underswap Toriel either.) Everything they did with it was controlled and calculated. So transferring their children’s SOULS between themself and Dings was very, very easy for them and something they did often to share the burden.
~ Sol