Instructions
Go to Madame Rummy's castle on Palaish Island.
Complete the Slime Challenge, which involves killing a number of Slimes in the gardens of the castle inside five minutes. Each time you do the Challenge the number of Slimes you need to beat goes up, to a maximum of 24.
Having completed the Challenge you are now allowed to access the Metal Menagerie.
Go to the first platform of the three inside the Metal Menagerie, and you will get to battle several Metal Slimes. The second platform has Liquid Metal Slimes, and the third contains a single Metal King Slime.
Battle as many Metal Slimes as you can in two and a half minutes -- after that time elapses, you are kicked out of the Menagerie and have to do the Challenge again to get back in.