Rock Portal Scissors
Win 3 co-op games of rock-paper-scissors in a row

Watch the video here: Rock Portal Scissors Achievement/Trophy Guide

This is a very easy achievement. To do this, of course you need a partner playing in co-op mode. Once you learn Rock-paper-scissors gesture, play the gesture with your friend. Just keep on doing this gesture, which may take several minutes until you win 3 in a row.

After this you should earn 20G points for the Xbox 360 and a Silver trophy for the PlayStation 3.

Watch the video here: Rock Portal Scissors Achievement/Trophy Guide