In order to complete this I could use some hints as to what to tweak and where.
Try making the enemies zig and zag on the way down. Create a new @velocity_x in the initialize method of the Enemy class, and then in the move method of the Enemy class you can change the x by @velocity_x. Now run your program - the enemies should move at a slant. Then make the enemies change direction with a small chance each frame, like when you hid the ruby in Whack A Ruby. This goes in the move method of the Enemy class as well. This is just one little idea of changes you could make to the motion of enemies.