This is not the fastest or cheepist way to Pack-a-Punch but it's easy and you should have a Pack-a-Punched gun before wave 10.
You need to open the 3 gas valves.
Blow open the hole.
Turn on the generator down the hole.
Open the Bunkers steel blast door.
Turn on the 2 power switches (timed)
Turn on first button to PAP machine.
Slide down the 3 sewer pipes and hit the button at the bottom.
Pack-a-Punch your favourite gun.