Letter Quest: Grimm's Journey Remastered

If you don't happen upon this achievement naturally while playing, it can be easily gotten in stage 18. Plague tiles are dark red with some black splotches, and when you use them they do zero damage, and if you don't use them they turn adjacent tiles into plague tiles.

In stage 18, the enemy will turn tiles into plague tiles often. When you get to him, continually hit to skip your turn until he turns all of your tiles to plague tiles. Plague tiles do disappear after a couple turns, but just keep at it until all of your tiles are plague tiles.