To unlock this, you have to beat the first 9 monsters - after each, you'll be presented with a comic book-style "back issue" to decode by cycling through letters. Once all 9 are decoded, the achievement will unlock.
The first few words of each issue are below to guide you on decoding if needed:
Issue 1:
Issue 2:
Issue 3:
Issue 4:
Issue 5:
Issue 6:
Issue 7:
Issue 8:
Issue 9: