Just, so You know, metal slimes are very rare and very difficult to kill. The may only have 4 (?) HP but their defense is SUPER high, even with the best sword in the game, you'll be lucky to do 1 point of damage. Also, they run away VERY easily, so they really aren't the XP gold mine that people assume they are. If you ever find one, just cross your fingers that you score a critical hit.
At higher levels (I think over 22 or so), you start to do noticeably more damage and can one-hit-kill Metal Slimes. Some people feel that this makes them valuable for leveling up in the later parts of the game.
But by the time you regularly do >1 damage/turn to metal slimes, you (1) can even defeat Charlock castle's enemies easy enough that that's a more effective place to train and (2) you should be strong enoguh to defeat the Dragonlord.