What did the girls from Usagi Team watch on Final Eve?
They were watching Kelly's Heroes.  It's an old war movie in 1970 about a group of American soldier who tried to rob a bank far behind enemy line.



While the original poster looks different, it match with Japanese poster for the movie




The scene they were watching is around 1:56:00 when Oddball and Kelly, the main character, shoot down a Tiger, the same tank with Kuromorimine's tank.





Notes: 


The girls seems to use the same tactic against Elefant, they manages to outmaneuver Elefant, and shoot it down from behind in a narrow road. Initially, they shouted, "Operation: Kelly's Heroes".
When Yukari infiltrated Saunders, she called herself "Sgt. Oddball, third class" from the "6th Armored Division." Which I believe came from Oddball from Kelly's Heroes