When starting out on Mission 12, you will notice a bunch of staircase 
platforms. Then wraiths will swarm at you from every corner. Slay all 
the wraiths and make your way to the middle stair platform to the barred 
door to your left. Charge up the Alternate Lightning Gun and juice the 
power stone in the wall. Walk inside and recover the Alternate Gatling 
Gun. If you don't have the Rifle, this will do for BIG DAMAGE.