If you're after completing this achievement ASAP, open the spoiler. If you are after enjoying the story without spoiler, just continue playing as you would and this will eventually unlock.
Things might be a little different for you as choices affect how the story goes.
*** Spoiler - click to reveal ***
This final chapter will all be about defeating the Admin. Follow the QTE's to damage the Admin as he takes you through several stages. Once he has blue balls above him and you take control over Jesse, head close to the admin and try to time and hit the balls back. If done correctly you have to rush over to him so you can attempt to damage him. Move away from him once he throws snowballs around. Hit him twice more to finish this stage. (I'd slay my sword when his arms started swinging and always hit the blue fireball first try). Afterwards you' ll have a swordfight with him. Just hit him and dodge once he attacks you. Again some more QTE's with dialogues afterwards. To finish it off, head over to the gauntlet before Romeo does. The building will then start to collapse. You can either choose to take Romeo with you or to leave him behind.
If you leave Romeo behind, he will lure the monster away during your escape, allowing you to make it to the portal.
If you take Romeo with you, Radar will show up with a giant enderman, which will kill the monster, allowing you to make it to the portal.
After some final dialogue you'll return to Beacontown, which is being rebuilt. First, you'll adopt Stella's llama. Afterwards, you get to choose between staying in Beacontown with your friends or to hit the road with Petra. If you leave with Petra, you can either make Radar the ruler of Beacontown or choose for a democracy.
That's it for Minecraft Story Mode Season 2. I hope my guide was useful. Congrats on your completion.