On Disc 2 right before you are ready to Fight the Demon Lord the final time go to Krage and fight monsters around this area. Here you will encounter Slimes, Red Slimes, Gold Slimes, Metal Slimes, Babbles, Metabbles, and MetlKings. To take out the weak monsters I prefer to use Vacuum to take them out easily. For Gold Slimes just fight them like you would regular monsters, and metal monsters use Evilslash to take them out in one hit.