This achievement has to be achieved in Crossword. There are usually about 3 words/clues which go almost from the entire left to the right.
here's a puzzle which I completed, here the words Glorioussublime, tremendoussuper and fantastickiller are all more than 10 letters, so completing any of those would have given me the achievement.
This puzzle is the first puzzle on crossword adventure for MAY 2017. You can simply start this puzzle and fill in either Glorioussublime, tremendoussuper or fantastickiller to get the achievement.