The first thing I did after beating the main story was fight Adel and recruit him to my party and then the same with Rosalin. They're both level 100 which shouldn't be much of a problem since you already took down the main story final boss. You can then go to the class world, warp back to the beginning (getting Prinny Mask in the process), try alternate versions (don't want to spoil it if you don't know about it but think Etna/Axel modes) and pass bills to get extra levels.|
A lot of the post game content like extra characters and some scenarios was DLC on the PS3 version so you could play it on that version if you pay for it. I played it on my mate's PS3 (and his copy of the game) before I had one of my own so I never really got to do the post game stuff before.
PSN ID: finn1