You'll start the game in a very dark room and you'll be informed that you've got six minutes before battery in your shotgun runs out of power. Sadly you won't be allowed to turn off the flashlight on your own and the time limit will include not only this level but the next mission as well. There will be an alternative, because you'll be allowed to use the flares. I would recommend that you focus on finishing both levels as quickly as possible, because flares are very uneffective and you can't use them with your gun simultaneously. Thankfully if you follow my instructions you shouldn't run out of time. Start off by proceeding to a corridor located to your left [#85].
Turn right and make a few steps forward to get to a much larger tunnel. Sadly this is where you'll encounter the first group of mutants [#86]. A single shotgun hit will be more than enough to kill them, however the monsters will keep respawning endlessly. As a result you should focus only on eliminating closest targets. You should also stay on the run in order to prevent the monsters from catching up to you.
Keep heading forward and remain inside the biggest tunnel. Ignore smaller tunnels encountered along the way and try not to fall down through one of the smaller holes, because you would only lose more time and ammo trying to return to the tunnel. Turn right once you've reached a large junction [#87]. Make sure to kill the beasts pursuing you from time to time, but don't spend too much time trying to eliminate them.
You will soon get to a new junction. This time you'll have to turn left [#88]. Remain on the main path and you'll soon notice flares in the distance. Keep heading towards these objects. Approach a large gate and use the valve to open the gate (you don't have to search for the valve anymore like you did in the first game). This will allow you to get to a new area of the sewers.
You may proceed to the next section. It should be much safer here, however don't forget to keep moving in order to prevent the monsters from catching up to you. Eventually you'll see a man running away from you. Head towards him, HOWEVER inspect a left corridor along the way. There's a cigarette pack (15 - New Mecca Day) here [#89].
Once you've collected the pack turn around and kill new beasts. If you don't see any - you're in luck. Go back to the junction and go to your left [#90]. Enter a new corridor and wait for the game to begin loading a new map.