The bottom level where Null is has a nonbreathable athmosphere so you should wear a gill, one can be found in a medicine cabinet in the Abandoned Complex.
Talk to Null, who asks you to kill all of the shoggoth spawn under the church in Lurleen.
If you agree, Null will give you a key to the armory, which is on the level above the level Null is on, stocked full of grenades and a grenade launcher. (Please note that this key will only work once, and will cease to work if you leave the base and the area resets even if you haven't claimed the explosives for yourself yet. The same goes if someone else already looted it.)
Head to Lurleen, to the south of the area is the church.
prchr guards the entrance to the depths underneath the church.
Sinful things like cigarettes, porn or drugs will make him attack you.
Kill a shoggoth in front of prchr to make him let you past him further into the church and get a journal entry.
Kill all of the shoggoth spawn.
Shoggoth spawn are most weak to burn, electric, and acid.
Return to Null after you have completed this, and you will recieve a 750 XP journal entry and a deathsuit.