Complete both of the Act 2 Armory missions.
2.1 A Call to Arms
2.2 The Handy Man
You must do this online and not in the Training mode (formally called Solo mode). I did it on the standard campaign mode but due to lack of players or possibly poor matchmaking I ended up doing both missions solo with 3 Bots.
Cooped Up Achievement.
*** Spoiler - click to reveal ***There is a secret to be found in A Call to Arms. Once you leave the parking garage, drop down go left and follow the street up to the last house, go round the left hand side of the house follow it round to the right. Keep following the fence that’s now on you left and in front of you you’ll see to chicken sheds, the secret is in the window of the first one. Which will get you the Cooped Up Achievement.
A Humerus Weapon Achievement.
*** Spoiler - click to reveal ***There is a second missable Achievement in The Handy man mission. You have to destroy several nests in this level all containing a different part of Bob. The final nest contains Bob’s arm. Once you have Bob’s arm and before exiting the level go around and kill at least 10 ridden to get the A Humerus Weapon Achievement.
At the time I played only the Cooped Up Achievement pop. The Bob’s Your Uncle and A Humerus Weapon only unlocked after I’d returned to the camp site.
Both levels are easy to complete on your own, it’s a mater of taking your time and not trying to rush through the levels, like the rest of the game it can be more challenging depending on what Corruption Cards you are up against.