I just wanted to let everyone know that Hostage saves in the campaign mode do NOT count toward the 50. You will unlock this trophy after saving 50 hostages in the "Hostage Arena" mode. (It doesn't matter whether you do Alpha, Bravo or Gamma...and the results are cumulative. (i.e. 15 in Alpha, 15 in Beta and 20 in Gamma.)
This is also a challenge so you will see the counter when you scroll through challenges on the menu screen.