bet you're trying to figure out what the Lupara is for this achievement...
its the double barrel (2 shot) shot gun.
In reality, the achievement is completing chapter 8 start to finish with the Lupara. There is a point half way where you will wind up back outside. This is still considered the sewer. Keep playing using the Lupara until the achievement unlocks.
If you get to the rat boss and do not have the achievement, you messed up somewhere. Technically, you are still in a sewer, but the rat boss is in chapter 9. restart chap 8 from the beginning.
this achievement is easy, but highly obnoxious since the Lupara only holds 2 shots, is slow to reload, and the level will diarrhea so may rat enemies you will want to use a better gun.
- yes you can use chapter select to get directly to the sewer (once unlocked. its chapter 8)
- set difficulty to easy
- its better to rush the level jumping over and running from enemies only using the Lupara to push back and scatter enemies if you can't.
- selecting 'restart checkpoint' will clear screen of enemies and does not cancel achievement. abuse this every time you get to a new checkpoint, it is extremely helpful.
- I don't know if sub-weapons cancel out the achievement. I did not use them. Didn't need them. (seriously, just rush the level and restart checkpoint if there are too many enemies)
addendum: previously there was a snarky comment here addressing the fact this achievement is straight forward and WTF are there so many down votes? do people not know what a sewer is? Apparently the thing throwing people off is the fact there's a very short bit of the level you briefly leave the sewer. Again, the achievement is completing all of chapter 8 with the Lupara. The outside bit is still considered "the sewer" so keep playing. Chapter 8 and 9 look nearly identical so the achievement should pop unexpectedly when 8 ends.