I am the defendant in a law suit. We are about to reach the Discovery End Date for the second time. i granted the Plaintiff an extension last time.

I assume that you have made a good faith effort to resolve the discovery dispute with the plaintiff which has failed.  You can file a motion to compel the discovery and/or to sanction the plaintiff (sanctions can go as far as dismissal of the complaint, if the offense is serious enough( for failure to comply with his/her/its discovery obligations.