...but damn are the battles slow and boring and it takes so long to gain a level. Really getting frustrated with this
Not going to lie, that was the main reason why I hated Dragon Quest for a while.
Haven't played the DS version so I don't know what level you need to be for the post game stuff but I did my best grinding on the island that has the entrance to the underworld in the NES version. Nothing like sicking Alena on any unfortunate Metal King Slimes foolish enough to draw near and watching the ensuing mess of critical hits flatten them.
Also, and I don't know why you wouldn't have done this already, but if you haven't cleaned out the casino for Metal King Shields for everybody who can equip them then you should. Remember, its a game. There's a save point nearby. Buy maybe 10 tokens, save whenever you end up in the black, and set a reset threshold once you have more than enough tokens to play for a five minute long losing streak so you don't actually have to play for more than five minutes before you realize you need to give up and try again.