This achievement requires first that you are playing a Casual puzzle with one other member of your team. You then must score a cumulative total of 50 bonus points by word-crossing with your teammate. To word-cross, complete a word that has a letter in it from a word that has already been filled in by your teammate. You will need to word-cross around 10 words, but the number seems vary and is hard to pin-point. This should easily be obtainable in a single game if you play a puzzle with a teammate, with your focus being on Up-Down words and your teammate's focus being on Across words. This achievement is a bit glitchy, so if it doesn't unlock just keep at it.