Yakuza: Like a Dragon

The name of this (and the 2 other job achievements) is actually misleading. Max Level for jobs is 99 but a job is apparently considered maxed out at Level 30. Likely because after this you don't get any special effects from leveling jobs anymore ( your stats do still rise though. There just aren't any moves or permanent bonuses to unlock after that)

You'll almost certainly achieve this one on your first playthrough, since you'll invariably have some character that you stick with the same job all the time in your party.

Most likely you'll either unlock it with Kasuga's hero job, or one of your female characters will unlock it with the idol job.

I kept everybody mostly with the same jobs all the time so by the time the credits rolled I had managed to get 5 characters to Job Level 30 with the remaining 2 at Level 28 and 29 respectively.
So even if you're going for the 7 Job achievement you shouldn't need to grind too much in Premium adventure.