Level: Creeper Woods, Soggy Swamp, Pumpkin Pastures, Redstone Mines, Cacti Canyon, HighBlock Halls (THERE IS A LOT HERE).
Recommended Tiers are: I, II, III, IV,
Tip: Are you struggling to kill the Enchanted mob? Damage it before the Book Illager is able to "enchant" it. It will be a lot easier to kill this way
An "Enchanted Mob" is a mob that is getting buffed from a "Book Illager".
The "Book Illager" (At least what I'm calling it) is an Illager that is holding a book, and will NOT attack the player. Instead it will "Enchant" an allied mob, this means it will make it larger, giving it more health, and sometimes a small damage boost (Enchanted Mobs are normally larger, have a beam hitting it and are a more greyish tone).
"Book Illagers" are extremely common and you will normally see them shoot a beam at an enemy mob this will sometimes cause the mob to be larger and have a slight greyish tone.
Kill the "Enchanted" mob (The mob the beam is hitting, again the mob should be greyish, or larger then the rest).
Repeat 50 times and the achievement is yours!