This achievement requires that you be in a Casual or Pro puzzle with at least one other player and may require a bit of communication with them. The other player first needs to place a Bomb power-up (it looks like a bomb) on the board. This will reveal a cluster of up to 9 letters across numerous words on the board. To unlock the achievement, you simply have to finish a word that contains any of the revealed letters. This achievement is probably best attempted with a teammate, as not many people typically use power-ups in normal play.