I spaghetti coded together a save editor that can replace all your festival plaza guests (and vips if you want) with ones that have Battle Agency sets. You can customize which sets they use or let them be chosen at random. You also can pick what grade they will be so that the Battle Agency becomes a balanced experience.
Feel free to create your own partner rosters (monotype, themed, no legendaries, only legendaries, etc.) and share them for others to use since I was too lazy to actually make one to go with this.
It probably works with citra. It worked on a file that had never gone online.
Note: PKHeX is required in order to use your save afterward. Also, I coded this in java so that is probably required too.
I also can't really help with troubleshooting since all I'm obligated to do is make sure it works on my machine.
Download link:
https://drive.google.com/file/d/1tF4RJFRI6o8Cv_aTzE-8c01GFbL2jldk/view 