
Once destroyed through the potion you'll never get it back.

The crucial info here is that Mighty Metamorphim appears to roll relics from the pool of relics you haven't found yet. If you have Masa or Mune then it will be destroyed and you will not be able to get it back. When you use a Mighty Metamorphim, it re-rolls all your relics on hand and makes you drop them. The important thing to note here is you DO NOT want to pick up either Masa or Mune, I will explain why. Sewing Kit not super useful to you anymore? Doesn't matter, grab it. Re-roll shops as much as you can to try and get Mighty Metamorphims and relics.

Basically you run through the game picking up as many relics as you can get your hands on, and you want singletons, so try to avoid combining your relics (bomb relics, boots, etc). You're going to want at least one for a chance, or in my case two to guarantee it. So make sure you have those unlocked as well. Ultimately what you're looking for are the Mighty Metamorphim potions. Instead, make sure you've got both unlocked and then put some other relic on the altar so both are in the relic pool. The problem here is the chances of finding Mune before you hit the one floor where the altar spawns seems to be really low, at least for me. The standard method is to put Masa on the altar and then come back in a later run with Mune.

You're going to want to be at end game with a bunch of extra gold to be able to buy anything you want from the shop etc. This is going to require a little bit of setup, but I assume if you're at the point of going for rare achievements you're in a good spot. This is one of the last 3 achievements for me (summoning stone 30 and die 100 times) so I've been going hard at it and I think I stumbled across a possibly slightly easier method of getting the Masamune than what's commonly suggested.
