I am not going to talk You thru the whole mission but am simply going to tell You of a way to spawn Gutter Runners !
As You play thru the mission the Gutter Runners spawn fairly sporadically and if , like My group , You get to the wagon having only killed 6 Gutter Runners - You then have to wait around a good 1/2 Hr or so to get the remainder for the achievement .
However ...
If You are standing at the final tome and look thru the big fence behind you ... You should see a big Bell on the right hand side .
To get to the bell from the tome head up the tunnel You are in and take the 1st right then take a right again and this will bring You to the bell.
If You go up to the bell You will see that You can ring it .... And its the ringing of this bell that will spawn in Gutter Runners !
The tome is at my feet ... This is a view of the fence !
This is the Gutter Runner spawning bell !
However be warned ...
1/ If You are playing in a group of 4 and You ring the bell You will get 4 Gutter Runners spawn ... If You are playing with 2 others then You will get 3 spawn & so forth ( 1 Gutter Runner per human player )
2/ Once You ring the bell Your amount of Gutter Runners spawn in virtually instantaneously.
3/ You will still be attacked by the other specials and normal rats.
4/ You can ring the bell as many times as You like .
Many thanks to Pnoid who told Us about the bell ... albeit after We had the achievement by hanging around at the wagon waiting for our remaining Gutter Runners to spawn !!