Spec Ops: Rangers Lead the Way (cheat)
Players: 1 
Invincibility and Reset Mission Time 
To become invulnerable, press ALT+SHIFT+V and access your inventory. You will have the viewmaster. Use it an the screen will flash blue, your ranger will jump and the time will reset giving you the time that the mission started with. 
Level Select 
To gain access to all missions you will have to move the Savedata.txt file out of the Spec Ops directory.