There are several Metal Slimes found throughout the game, each increasing in EXP given. There are many ways listed on sites to beat them. For the first and weakest MS, Metal Slash is the best option as you find them early in the game. However, I suggest investing in either Spear or Axe skills. Thunder Thrust (spear) and Hatchet Man (axe) will always critical, killing any Metal Slime even the Platinum King Jewel (PKJ), but the chance of hitting with these skills are quite low. Overall, these two skills are the best for hunting Metal Slimes.

Metal Slime name EXP given
Metal Slime 4 000
Metal Medley

12 000

Liquid Metal Slime 40 000
Metal King Slime 120 000
Platinum King Jewel 240 000

MKS and PKJ can only reliably be found in high level grottoes. Check the bestiary on GameFaqs for help. Check the grotto board to help find which grottoes are worth searching through.