You can get this by pretty much killing every shade in sight, from my knowledge they’re unlocked from killing shades
A good place to kill a lot is the northern plains when it is overcast
Another place I’ve found to have a lot of shades is also the laboratory
I also found replaying Nier’s Dream from “The Magic Stone” very useful and probably the easiest way to get words
There are 120 words in total meaning that you have to collect 60 to unlock the achievement