There are 3 rows of 5 letters in each battle. You must use all 5 letters in a single row to make a single word. You MAY use extra letters to make 6, 7 or 8 letter words. As long as you use al 5 letters in a single row (left to right) it will count.
Do this 3 total times for this achievement. These do NOT have to be done in a single battle but over the entire time you play the game, even across multiple playthroughs.