This achievement unlocks after achieving the completion of the first six chapters:
Below will be a walkthrough video: of the whole Episode. If you feel stuck at some point along the way, feel free to check out the video:
Credit to NRMgamingHD.
Also, here is a descriptive walkthrough of the last chapter:
*** Spoiler - click to reveal ***Time for the aftermath. There will be a bunch of drama, some comedy, and a healthy dose of BS. Then, everyone will be back together again. After a big ol' hug with midget Obi-Wan, the lesbian space Australian, and the rest of the group, you will get a wedding invite. All of your friends go to get some loot, but Gortys will stay for some conversation. Your robotic friends will go off to talk and a race to the loot will begin. Now, the two playable characters can talk about Sasha and Rhys' relationship and you can go ahead with either shutting it down or making it happen (in a way). Enter the vault and talk for a long while on both ends of the conversation, then open the chest as both characters...