Select the Area 51 Challenge. Start by killing everything in sight. Wait for the 
big metal robot to appear from the right. Do not destroy it. It does not kill you. 
As soon as it enters the screen, stand directly under it and start shooting in the 
opposite direction. You will kill everything that appears from the left side and 
the enemies jumping over the robot with ease. Keep this up for about twenty minutes 
to unlock both achievements at the same time.