This is story related and you can't miss it - it's essentially a horde-like game where you kill ODs before they destroy the vat. Just stay high, and keep using your Flaming Compensator for most of your kills. When the big guy comes in, set him on fire with the Flaming Compensator and then let him have it with the Dirty Harry... key thing is to kill ODs before they get to the vat.
Also, you WILL run out of ammo, so make sure you pick up boxes of ammo...