Here are the assembly instructions for the Reaver heavy weapon support trooper found in Bag 2 - there are two of these in the box game. The Reaver support weapon and the left arm, are supplied attached to a sprue - so these need to be cut off the sprue before assembly. Be careful to cut away the sprue marks on the plug-joint of the arm - to ensure it fit snugly in the socket-joint of the shoulder.
Here are the parts -
The model is simple to assemble - attach the right arms first, and then the gun. The arm fits solidly to the shoulder (it only fits one way), ensuring the gun fit is accurate.
Here are the stage by stage pictures of the build -