The first time you can find Metal Slimes in Dragon Quest III: HD-2D Remake will most likely be on the fifth floor of the Tower of Transcendence, which is around the halfway point of the game.
While you can level up anywhere in Dragon Quest 3 HD-2D Remake, the best places include Metal Slimes/Liquid Metal Slimes. They give you the most bang for your buck when it comes to leveling up.