There are 3 way to do this
1.do level 2 and get the scrap with 3200 the save and quit and repeat the whole thing over and over
2.do level 3 and when your at the elevator you have to destroy dont destroy it kill all enemys and hide under the stairs.after when 2 norts come out of the elevator sneak up,kill move them and salvage them kepp repeting this over and over and you get lots o salvage
3.do the mountain pass mission and when you get to the hoppa pad dont destroy it.get the distracted by sentry gun or holdecoy then hide behind the hoppa pad and kill move em and salvage em all.keep doing this and you get lots of salvage