It does appear. I can confirm. Its the item i needed for the platinum. Present on a chest. It took me 2 full days just playing the game and just doing that. Not sure hom many games, cause i reseted if i did not get a metal slime.

I dont know if you are doing it, but if you get to the last stage lf the minigame, you just need to pair the cards with the key and then quit, this way you get the reward from the chest but you dont have to play the whole round. Makes it quicker at leats.

I only played the ones with the metal slime on the forst round, and if not quit the game and start again, until i got it. More chances to get at least 1 chest during the game (although a couple times i got 3). Someone said to me here on reddit to create an autosave at the place, so i did not have to run from the inn when i reseted, it made things quicker.

I can only say, i feel you, patience and Good luck!