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.