In Dragon Quest IX: Sentinels of the Starry Skies, there are a couple places that are considerably better for grinding experience than others.
Toward the beginning of the game, go to Quarentomb and fight Metal Slimes. Keep them stunned if possible to keep them from running and you can easily net about 800 to 1000 experience per kill depending on the level of the slime.
Later in the game look for Liquid Metal Slimes. Before you beat the game, Bowhole will be the best place to find Liquid Metal Slimes. After you’ve beaten the game and can fly around the world map, the best place to find Liquid Metal Slimes is at Slime Hill north of Angel Falls, but it’s only reachable by flying.
Liquid Metal Slimes will give you 40,000 experience per kill, but they like to run from you, especially if you’re higher level. The best technique to kill them is to stun them if you can, or to use abilities that either boost your critical chance or that give you more than one hit in a single attack.
Thunder Thrust with spears gives you a 50% chance of a critical hit, Hatchet Man with Axes has a similar effect. Claws have a four attack ability, and Falcon Blade used with a Swords gives you two attacks, etc.