If you are not sure how to create an action, see the FAQs
Also: get a column in the files list that shows the genre (it's there by default so unless you have removed it, you don't have to do anything special here).
Sort by the column for genre.
Now apply a filter
NOT %genre% HAS :
The files in the file list should now be reduced to a files that do not have a colon in the string - which should be the case for every single-word-genre.
The sorted genre list should now start with
Select all the tracks that have Bassoon as genre and apply the action "Genre#Instrument".
The selected files should now show as genre:
Instrument: Bassoon
Press F3 2x to refresh the filtered list - the files with Instrument: Bassoon should not be in the file list any more.
Next, the cello ones would have to be treated. And as Cello and Cello Concertos are probably close to each other, you can select all the files with Cello in it in 1 go and then apply the action Genre#Instrument.
After 2x F3 Choral would be next, the proceedings are the same: select files BUT this time execute the action Genre#Real Genre.
Go through the whole list of genres until the file list with applied filter stays empty.
The quickest way would be to select as many files of the same type (genre or instrument) in 1 go as possible and then apply the corresponding action.
So you can safely select all files with genres starting with "Ethnic" and apply "Genre#Real Genrres".