I fought around outside the Dark Ruins, and in Argonia until I was strong enough to get through the ruins - and then I levelled up like crazy in there, because Dhoulmagus was (for me, at least), a really, really difficult boss.
Where else is there...? The Dragon Graveyard is good, if you're strong enough to handle it. It's down in the middle of the desert on the southern end of the same continent that Argonia is on. They've got King Metal Slimes in there (extremely rarely though, and they run away like chickens, so you have to be patient and frustrated for a while) which dish out 30000 EXP when defeated. The other enemies are pretty good, too. I don't know if you can actually get into the Graveyard without doing the Cash and Carrie subquest, though. Go have a look.
If you have the patience (I know that levelling up takes waaay too much time fighting against normal enemies in DQ), try to get Angelo up to lv. 30, where he gains Multiheal. It'll be infinitely useful against Dhoulmagus, and I regretted not having it.
I haven't met Cash and Carrie yet, though. How do I get that sidequest started?
Unfortunately, I don't think you can start that sidequest until after Dark Ruins (they're in the mansion in Baccarat). But like I said - perhaps I can get into the Dragon Graveyard before the sidequest. I'm not sure, so you'll have to check.
I just thought of something else - if you take your handy-dandy ship and sail to the southern end of the Argonia-continent, and keep sailing south, into the fog, you will eventually come to an island (very small) that isn't on your map. There, you can randomly encounter Liquid Metal Slimes - they might not give 30000 EXP, like the King Metal Slimes, but they give you 10000 EXP, which is pretty handy.
You should consider investing 7 points into Jessica's staff skillset for Acceleratle and Kasap. Those two spells should prove rather useful. Metal slimes are always a good way to go, but you may have some difficulties with keeping Liquid Metals around long enough (60 pts. in Spears or Axes will give a crit or nothing ability which will make short work of any metal slime).
The beach near the Riverside Chapel has metal slimes on it. The bastards are hard to beat because they run off within two rounds and they're crazily damage resistant. Plus they're sort of an uncommon encounter. "Buff"-ing Angelo and using his Metal slash is a good way to beat them, also if you have the Hero equiped with the spear Lightening Thrust is a good way to defeat them as well. Metal slimes aren't exactly dangerous but they give a LOT of EXP.
One of my FAVORITE games, and, in fact, is one of the inspirations for some of ENGINE. It took my 120 hours to finish it, and I still had a few things I never was able to do at the very end in the Dragon world. ugh