First, build a Zombie shield. Click this link on how to do that.
Second, we need Tomahawk. Here’s the link for that on how to.
Third, go to Warden’s Office straight on the perk machine and use your window shield to see the hidden numbers. This number will be on the three post.
Fourth, charge your shield then go to the spiral staircase. Blast your shield on the number Pads and put the number. If you did this right, you will hear a quote on your character while the elevator drop.
Last, go to Docks. There, you will see a Volt Meter. Blast that with your shield the wait for the crane. Once the crane stop, use your tomahawk and throw it on the net. You will something fell. Go to the location where it drops and get that.
Note: don’t wait too long to throw your tomahawk because the crane will be back again to where he at.