Is there a better way to level up in Dragon Quest IX?

Unfortunately, killing metal slimes is your best bet. They're fairly rare, but they're also worth big, big XP. The Quarantomb has a fair number of them, and that's where I farmed in the early part of the game. You'll want to go prepared with attacks that are strong against Metal enemies, plus high speed and speed boosting spells. There's a good strategy here that may help save you some trouble.

This GameFAQs thread lists some locations where you can find other metal slime variants later in the game. The consensus seems to be that Bad Cave is the best place to farm Liquid Metal slimes.

It may feel grindy, and that's because it is :( All the DQ games I've played have been this way - "it's a feature, not a bug." :)

Since you've already made it to Gittingham Palace, don't waste your time with metal slimes at the Quarantomb. You'd be better off fighting regular monsters. What you should be hunting are Liquid Metal Slimes. They give 40,000 xp or 10,000 xp per character if you have a team of 4. You can find them on the 3rd floor in the Bowhole. Make sure you have someone who knows Hatchet Man or Lightning Thrust(axe or spear)... those moves have a 50% chance of hitting, and if they hit, it's an automatic critical.

what you need to do is go to the gittingham palace and hunt around for lethel armours untill thay send for back up which are cure slimes and keep defeating them but make sure you have at least 2 cure slimes and the lethal amours send them up and if you go from 1 - 16 cure slime and do it 4 time u will gat about 90.000 exp