This is for completing all maps in act 3. This can be done in online or offline mode. Before the update you could only gain achievements in online play only but since an update you can now get achievements in offline mode.
The acts and chapters are as follows. Once you complete the last one the achievements will pop.
Act 3 – Farther Afield
Dr. Rogers Neighborhood
Farther Afield
Blazing Trails
Cabins by the Lake
Garden Party
T-5
Remnants
A Friend in Need
Making the Grade
The Road to Hell
The Body Dump