Making Text Shadow using opacity mask in Illustrator.

Your final artwork will be like below.

First take a new document and take the rectangle tool.

Then take the type tool and writhe something.

 Then go to TYPE > CREATE OUTLINES. Make a copy of the text and right click on it. Go to TRANSFORM > REFLECT > HORIZONTAL.

 Then move it to just below the original text.

 Select the lower text and open the transparency pallet. Double click on the blank space just right side of the icon of the element.

You will look that the lower text is hidden now. No tension! Take the rectangle tool (color=white) and make a box around your hidden text. Your text will be appeared again.

 Fill the box with linear gradient (black to white). You can also lower the opacity of the black bucket.

 Again go to the transparency pallet and lower the opacity to 50%. Double click on the left icon of the mask pallet.

Select both text files and group by pressing CTRL + G. Make some copy of the grouped object.Your work like below.