You will need a tool that artificially changes player count to 4, tricking the game into believing you play in a full Co-Op squad, thus rewarding you the achievement upon completing the mission(s). It's not my creation, I found it here. Tool is made in late 2014 as an update to existing B1/B2 tool, to work with the Pre-Sequel.

Apparently the added difficulty also grants better loot drops but I didn't bother to verify this. Meaning you could continue playing with it to ease the grind if you so desire.

1. Download the tool[1drv.ms] (inside the .zip).
   
2. Run game, select desired character, click Continue (crucial!).
   
3. Alt+Tab to desktop and run the tool. Click Scan and move slider all the way to the right.
   
4. Alt+Tab back into the game and Start the "Sub-Level 13" mission(s) in Outlands Canyon.
   Couple of people reported it is sufficient to have the tool activated just before turning in 2nd (last) mission and still get the achievement.

You'll know it's working when you get a 5 second timer during map transitions and being able to pick up ammo even if you are maxed out. Finish it and the achievement will pop out. To stop the tool player scaling effect close the game completely as well as tool itself.