"a far stronger showing of necessity and unavailability by other means" than is needed to justify discovery of fact-based work product. Id. (quoting Upiohn Co. v. United States 14 EFTA_R1_0 1293449 EFTA02335911 Case 9:08-cv-80736-KAM Document 330 Entered on FLSD Docket 07/06/2015 Page 15 of 51 449 U.S.