The “smoking gun” document does not do you any good unless you can find a way to get it introduced in to evidence. You have to determine what foundation must be established to introduce the document into evidence, call the witness or witnesses necessary to establish that foundation, and be able to link the document to your theory of the case as set forth in the complaint (to establish that it is relevant).
Here is an article by Leslie O’Tool and Wendy Sexton, two defense lawyers, which explains how your opponents intend to try to keep that smoking gun document out of evidence.
The article appeared in the Fall, 2005 edition of FDCC Quarterly, PDF page 79.