In Chapter Five, you'll obtain the Job Change option at Hello Work. Here, you can change your job over to anything else you have the experience to unlock. By that, I mean there are certain criterion to satisfy some jobs. For Kasuga, he will need a specific level and/or a stat level to change, while everyone else may need a level and/or a bond level to change.
For this trophy, you can simply change everyone over to one new job, find a group roaming near the building and defeat them with your new abilities, and then return back to your current setup. Be careful when switching up jobs, as they have their own job levels, meaning it will take longer to max one out if you keep changing it up.