A Tetris is when 4 lines are cleared at once. To do this, you must build leaving a sport just for the stick on either the right or left hand side.
The fastest way to unlock this achievement is to start a new game on marathan, when you receive a stick, hoard it with (RB). That will save it until you need it. continue playing until you have at least 8 lines ready to be cleared and you get another stick from the game. Use the stick you just got to clear out 4 lines (a tetris), then as the next peice comes down swap it with the stick you have saved by pressing (RB). Acquire another tetris, and achievement unlock.