You can get this about midway through the 7th mission called "Bunker". The difficulty doesn't really matter but fighting the Trigens (the mutants) later is of course easier on the easy difficulty.
After fighting through some halls at the beginning you will go down through the bunker. You will get to a short outdoor are where you're walking across some walkways. You will enter the bunker again for a couple of rooms before you get to the outdoor walkways again. After you walked across the second set of walkways you'll enter a section of the bunker where you can see a couple of mercenaries in a room with some cages. Doyle will also mention that you can release the Trigens.
Do so walk up the stairs on your left and you can see the lever up there. Just pull it by holding X and voilà!