@sheepdisease, do you know that you can select multiple files at once, enter the desired value on the Tag Panel and save the change, effectively changing, e.g., the artist for multiple files at once?
If you already know this and want to perform this change inside an action group, you can use an action "Format value" for field ARTIST (and ALBUMARTIST respectively).
@ohrenkino has already pointed this out above, but I think it got lost in all the other advice he was trying to give.
If you want to do yourself a favor, you can also read the help topic on Actions in Mp3tag and the FAQ: Actions and batch-operations. They give a pretty good overview on the different action types and what you can achieve with them.