In Dragon Quest IX: Sentinels of the Starry Skies, liquid metal slimes are essentially the best experience grinding monsters in the game, giving you 40,000 experience per kill.
Before you beat the game, Bowhole will be the best place to find Liquid Metal Slimes. Go down to B3, and look for the stairs to B4. Go down to B4 and come back up to B3 to reset everything. Stay in the little nook to the right of the stairs, and just sit there and wait for a Liquid Metal Slime to walk by to attack it.
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 great experience if you can kill them, 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.