During the run prepare a 4/4 Tungsten Coating, 4/4 Gyro Staibilizer and 2/4 Shield Generator (or alternatively lock-on extender). Using this in the second half of the game is probably faster than continueing running fast as checkpoints will require you to destroy/kill stuff.

Riot Shotgun does by far the most damage, The engineers fight takes not more than one clip if everything hits from close range (around 20s).

As for Salvador, getting him stuck inside the room prevents him from knocking you back very far, which optimizes your shotgun hits. You can hit him quick enough so he never does anything (though I haven't managed that yet) and you can shoot him once from close range immediately after Grapple.

Kill is at 1:15

You get the swag bonus of getting dialogue during the curscene/credits because dialogue doesn't trigger until Salvador leaves the room.