Iron Crypticle

This achievement requires you to collect all five BONUS letters, which look like runic scrabble tiles. These are completely random drops, and you can get duplicate letters dropped too. However, since there are only five, your chances of getting all five on a run are pretty decent. I collected all five BONUS letters at least three separate times during my time with this game, and none of those times required me to buy any from shops. If you aren't having any luck, you can buy a "strange rune" from the shop to add to your letters.